Starling Accounting Won't Link to HMRC for MTD? Here's Why and How to Fix It

A Reddit user recently posted a frustration that is echoing across small business forums: they’d signed up to Starling Bank’s free accounting software, tried to link it to Making Tax Digital with HMRC, and kept getting rejected. Worse, the accounting tab had since “reverted to a foreign language/strange code” they couldn’t understand. The comments confirmed they were not alone — multiple users reported the same MTD link failure, the same “save business failed, please try again” error, and the same loss of features after Starling replaced its old Business Toolkit with the new Starling Accounting platform.

If you’re hitting the same wall, here’s the good news: the problem is almost certainly not Starling’s software. It is the HMRC authorisation step — and it is fixable. Here’s what is actually happening, why, and how to get the link working.

What Starling Accounting Actually Is (and Why This Is Happening Now)

First, some context that explains why these issues are surfacing in 2025. Starling’s old bookkeeping tool, Business Toolkit, was a lightweight £7/month add-on. In 2024, Starling acquired tax and bookkeeping software company Ember specifically to build a more robust MTD-ready product. The result is Starling Accounting, which replaced Business Toolkit and includes a free, HMRC-recognised MTD for Income Tax tool.

The migration from Toolkit to the new platform has not been smooth. Users have reported:

• Lost invoice matches (all manually matched invoices from years of Toolkit use were unmatched in the new system)
• Missing features (bill upload/payment, receipt attachment, multi-invoice payment matching)
• Display bugs (the “strange code” the Reddit poster saw)
• The MTD link failing repeatedly

Some of these are Starling’s responsibility and they have acknowledged they are “working on” the feature gaps. But the MTD link failure is a different beast — and it is almost always caused by something on the HMRC side that you can fix yourself.

How the MTD Authorisation Actually Works

Understanding the mechanism makes the fixes make sense. When you click “link to HMRC” in Starling Accounting, it does not just send your data over. It initiates an OAuth 2.0 authorisation flow — the same technology that lets you “log in with Google” on a third-party app. Here is what happens:

1. Starling asks HMRC for permission to connect to your tax record.
2. HMRC redirects you to a Government Gateway login page.
3. You log in, proving you are who you say you are.
4. HMRC checks that your account has an active MTD for Income Tax enrolment.
5. HMRC checks that the National Insurance number Starling sent matches the one on your Government Gateway account.
6. If everything matches, HMRC sends a token back to Starling, and the link is established.

The token lasts 18 months, after which you have to re-authorise. Your software should remind you when it is time to reconnect, per HMRC’s guidance on authorising software.

When the link fails, it fails at one of steps 3, 4, or 5. Here are the specific causes, in order of how common they are, and how to fix each one.

Fix 1: You Have Not Signed Up for MTD with HMRC Yet

This is the single most common cause, and it is the one nobody tells you about. Signing up for MTD for Income Tax with HMRC and authorising your software are two separate steps. You must do the first before the second will work.

As HMRC’s sign-up guidance explains, you sign up for MTD through HMRC’s online service using the same user ID and password you got when you registered for Self Assessment. After signing up, it can take up to 72 hours for HMRC to process your enrolment. If you try to authorise your software before that processing is complete, the connection will fail — because HMRC does not yet have an MTD enrolment to link the software to.

How to check if your sign-up has processed

1. Log in to your HMRC online services account.
2. Look for a Making Tax Digital for Income Tax section.
3. If you see it, click through — you should see your UTR, upcoming submission deadlines, and a “Your reporting obligations” panel listing MTD for Income Tax.
4. If you do not see it, your sign-up has not processed yet. Wait the full 72 hours, then check again.

If 72 hours have passed and it is still not there, contact HMRC’s MTD helpline. Do not keep retrying the Starling link — it will keep failing until the enrolment is active.

Do you actually need to be in MTD yet?

MTD for Income Tax becomes mandatory in stages:

• April 2026: sole traders and landlords with gross income over £50,000
• April 2027: income over £30,000
• April 2028: income over £20,000

You can sign up voluntarily before your mandatory date — and Starling’s MTD tool is free, so there is no cost barrier. But if you are not yet required to use MTD and you are not ready to commit to quarterly updates, you do not have to link anything yet. You can keep using Starling Accounting for bookkeeping and file a normal Self Assessment return by 31 January.

Fix 2: Wrong Government Gateway Credentials

The second most common cause. When the HMRC authorisation page pops up, you need to log in with the same Government Gateway user ID and password you used when you registered for Self Assessment. Not a new one. Not your GOV.UK One Login. Not a different Government Gateway account you created for something else.

This trips people up because HMRC is in the middle of migrating from Government Gateway to GOV.UK One Login, and the two systems coexist. Here is the rule, per HMRC’s guidance:

• If you sign in to Self Assessment using Government Gateway (your user ID is up to 12 digits): use those same details in your software to connect to HMRC.
• If you sign in to Self Assessment using GOV.UK One Login (your username is your email address): you need to create a unique Government Gateway user ID and password specifically for commercial software authorisation. This is done from within your HMRC online account — go to Profile and settings → Manage sign-in details → View next to “Commercial software guidance” → Create Government Gateway details. This new credential cannot be used to access HMRC online services directly; it only works for software authorisation.

The key mistake is creating a brand new Government Gateway account when you already have one with Self Assessment enrolment. If you do that, the new account has no MTD enrolment attached to it, and the authorisation will fail with a “client or agent not authorised” error — the same HTTP 403 error that MTD software vendors report as the most common failure.

If you have lost your Government Gateway credentials

If you cannot find your user ID or password, recover them before attempting the Starling link. Go to the Government Gateway sign-in page and use the recovery options. If you are completely locked out, see our guide on what to do when you’re locked out of Government Gateway.

Fix 3: National Insurance Number Mismatch

This is the one Starling’s own support bot flagged in the Reddit thread — and it was right to. When Starling sends your details to HMRC for authorisation, it includes your National Insurance number (NINO). HMRC checks that this NINO matches the one on your Government Gateway account. If they do not match, the authorisation fails.

This is a documented issue across all MTD software, not just Starling. The VT Software knowledge base describes the exact error: “According to HMRC, the National insurance number set up in VT is not associated with the government gateway user ID used to grant authority.”

How to fix it

1. Check the NINO in Starling Accounting — go to your business profile settings and verify the NI number is entered correctly. The format should be 2 letters, 6 digits, 1 letter (e.g. AB123456C). No spaces.
2. Check the NINO HMRC holds — log in to your Personal Tax Account and check the NI number on file.
3. If they do not match, correct the one that is wrong. If Starling’s is wrong, update it in the app. If HMRC’s is wrong (rare, but it happens), you need to call HMRC to have them correct their records.
4. Do not copy and paste the NINO — several software vendors recommend typing it in manually, as copy-paste can introduce invisible characters or formatting that breaks the match.

The “two accounts” problem

One Reddit user mentioned they have two Starling accounts but can only link one to MTD. This may be because MTD for Income Tax is linked to you as an individual (via your NINO and UTR), not to a specific bank account. If you have two sole trader businesses, they both sit under your single MTD enrolment. If Starling’s software only supports linking one business to MTD at a time, that is a Starling limitation — contact their support about managing multiple businesses within the accounting tool.

Fix 4: The Authorisation Timed Out

A smaller but real cause. The HMRC authorisation window expires after 10 minutes. If you start the link process, then go searching for your Government Gateway credentials or get distracted, the session times out and the link fails — often with an unhelpful generic error.

Have everything ready before you click “connect to HMRC”:

• Your Government Gateway user ID and password
• Your UTR (10 digits, found on any Self Assessment correspondence)
• Your NI number

Then complete the authorisation in one sitting.

The Strange Code in the Accounting Tab

This is a separate issue from the MTD link failure, and it is Starling’s problem to fix. Multiple users have reported the accounting tab displaying raw code or unrendered text after the Business Toolkit migration. This is a display bug — a rendering failure in the interface, not a data corruption issue. Your actual transactions are not affected.

What to do

1. Report it to Starling support via in-app chat and ask them to escalate to the accounting team. The Reddit poster has already done this — the more reports Starling receives, the faster they prioritise a fix.
2. Try clearing the app cache / reinstalling — sometimes display bugs are cached client-side and a fresh install resolves them.
3. Check on the web version — if the app is broken, the web banking interface may render the accounting tab correctly.
4. Do not attempt any “fixes” involving your actual data — this is a UI bug, not a data problem. Do not delete transactions, re-match invoices, or rebuild your records in response to a display glitch.

The Missing Features: What You Can Do Now

The bigger frustration for many users is not the MTD link — it is that the new Starling Accounting has lost functionality that the old Business Toolkit had. Specifically:

• Uploading and paying bills — not available in the new system
• Attaching receipts to transactions — not available
• Matching one payment to multiple invoices — confusing or broken in the new system
• Previously reconciled transactions — unmatched during migration

Starling has acknowledged these gaps in webinars and says they are “working on” them. But if you need these features now, you have options:

Option 1: Use Starling Accounting alongside another tool

Starling integrates with Xero, QuickBooks, and FreeAgent. If you link your Starling account to one of these, you get full bookkeeping functionality (bill management, receipt attachment, multi-invoice matching) while keeping your Starling bank feed. This costs money — FreeAgent is free with a NatWest or Mettle account but not with Starling; Xero and QuickBooks charge monthly fees — but those fees are an allowable business expense.

Option 2: Wait for Starling to add the features

Starling has said more features are “coming soon.” If your bookkeeping needs are simple (basic income/expense tracking, invoicing, and MTD submissions), the current Starling Accounting may be enough. But if you need bill management or receipt attachment now, waiting is not practical.

Option 3: Switch to a different business account with built-in accounting

If the Starling Accounting gaps are a dealbreaker, it may be worth moving your business banking to a provider whose accounting features are more mature. Tide offers a free mobile-first business account with in-app invoicing and integration with Xero and QuickBooks — so you can keep your bank and bookkeeping connected without relying on Starling’s still-developing accounting tool. Other alternatives include Mettle (free, from NatWest, with built-in FreeAgent integration) and traditional banks paired with standalone accounting software.

Option 4: Use a dedicated MTD-compatible tool

If you are in the MTD mandatory group (turnover over £50,000 from April 2026) and you need full bookkeeping features, consider switching to a dedicated accounting platform. See our MTD software comparison guide for the options, and our MTD software costs guide for what is deductible.

Step-by-Step: Getting the Starling MTD Link Working

Here is the complete sequence, in order, to get the connection established:

1. Check whether you need MTD yet. If your turnover is under £50,000 and you do not want to start quarterly updates, you do not need to link anything. Skip to the next section.
2. Sign up for MTD for Income Tax with HMRC. Go to HMRC’s sign-up page and complete the sign-up using your existing Self Assessment Government Gateway credentials.
3. Wait up to 72 hours. HMRC needs time to process your enrolment. Do not retry the Starling link during this window.
4. Verify your enrolment is active. Log in to your HMRC online account and confirm you can see the Making Tax Digital for Income Tax section.
5. Gather your credentials. Have your Government Gateway user ID, password, UTR, and NI number ready.
6. Check your NI number in Starling. Verify it matches what HMRC holds — format is 2 letters, 6 digits, 1 letter, no spaces.
7. Initiate the link in Starling Accounting. Go to the Accounting tab → MTD section → connect to HMRC.
8. Complete the authorisation in one sitting. Log in to Government Gateway when prompted, approve the connection, and do not let the 10-minute window expire.
9. If it fails, work through the fixes above in order — sign-up not processed, wrong credentials, NINO mismatch, timeout.
10. If all fixes fail, contact HMRC’s MTD helpline. The issue may be on HMRC’s side (a known records issue), and they can run a check on your account. Starling support can also escalate, but HMRC can resolve record-side problems that Starling cannot.

What Not to Do

• Do not keep retrying the link 30 times. One Reddit user said they “tried 30 times.” If the authorisation is failing for a structural reason (no MTD enrolment, wrong credentials, NINO mismatch), retrying will not fix it — it will just frustrate you. Diagnose the cause first.
• Do not create a new Government Gateway account. If you already have one with Self Assessment enrolment, a new one will not have the MTD enrolment attached and will fail. Recover your existing credentials instead.
• Do not assume it is Starling’s fault. The MTD authorisation flow is HMRC’s system. Starling is just the software initiating the request. The error messages are often generic because HMRC returns them, not Starling.
• Do not ignore the 72-hour processing window. If you signed up for MTD today, the link will not work today. Wait, verify, then try.
• Do not abandon bookkeeping while you wait for the fix. Even if the MTD link is not working, you still need to keep records. Use Starling Accounting for what it can do, or a spreadsheet, or another tool — just do not stop recording income and expenses.

The Bottom Line

1. The MTD link failure is almost always HMRC-side, not Starling-side. The authorisation flow is HMRC’s system, and the most common causes are fixable by you.
2. Sign up for MTD with HMRC first, wait 72 hours, then link your software. This is the prerequisite that most people miss. Without an active MTD enrolment, no software can connect.
3. Use the right Government Gateway credentials — the same ones you registered for Self Assessment with. If you use GOV.UK One Login, create a unique Government Gateway ID for software authorisation.
4. Check your National Insurance number matches between Starling and HMRC. Type it manually, do not copy-paste.
5. The strange code in the accounting tab is a display bug from the Business Toolkit migration — report it to Starling, but it does not affect your data.
6. The missing features (bills, receipts, multi-invoice matching) are real gaps in the new Starling Accounting. If you need them now, link Starling to Xero, QuickBooks, or FreeAgent, or switch to a dedicated accounting platform.
7. Do not retry blindly. Diagnose the cause, fix it, then link once. If everything checks out and it still fails, contact HMRC’s MTD helpline — the issue may be a records problem on their side.

For the complete MTD roadmap, see our Making Tax Digital guide. For the full guide to choosing MTD software — digital links, spreadsheets vs cloud, free vs paid — see our MTD software guide. For the full guide to how MTD quarterly updates work once your software is linked, see our MTD quarterly updates guide. For help with Government Gateway access issues, see what to do when you’re locked out. For a comparison of MTD-compatible software options, see our MTD software comparison.

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