HMRC Rejected Your CT600? Common Errors and How to Fix Them

HMRC Rejected Your CT600? Common Errors and How to Fix Them

You've spent hours preparing your company's Corporation Tax return, hit submit — and HMRC sends it straight back with an error. Don't panic. CT600 rejections are common, and most are caused by a handful of fixable issues.

This guide covers the most frequent rejection reasons, explains the cryptic error messages HMRC sends, and walks you through fixing and resubmitting each one.

How CT600 Rejection Works

When you submit a CT600 through the HMRC Gateway, it goes through several layers of validation before being accepted:

1. XML Schema Validation — checks the file structure is valid
2. Business Validation Rules (BVR) — checks the data makes logical sense
3. Gateway Authentication — verifies your credentials and authorisation
4. iXBRL Validation — checks the attached accounts and computation

If any layer fails, HMRC returns an error response with one or more error codes. Your filing is not accepted — it's as if you never submitted it. This is important because your filing deadline continues to count down.

The Most Common Rejection Errors

1. BVR Errors — Business Validation Rule Failures

BVR errors are the most frequent cause of CT600 rejections. HMRC runs over 100 validation rules that check whether your numbers are internally consistent.

Common BVR errors include:

"CT600 period dates do not match accounts period"

Your CT600 shows one accounting period (e.g., 1 April 2024 to 31 March 2025) but the iXBRL accounts show different dates.

Fix: Check that the period start date, period end date, and balance sheet date are identical across:

• The CT600 form
• The iXBRL accounts
• The iXBRL tax computation

Even a one-day difference will cause rejection. If your accounts run to 31 March but you entered 30 March on the CT600, that's enough to fail.

"Tax computation does not agree with CT600"

The taxable profit in your tax computation doesn't match the figure in Box 435 (or equivalent) of the CT600. Our CT600 box-by-box guide explains what each box should contain.

Fix: Recalculate from the tax computation through to the CT600 boxes. Common causes:

• A rounding difference (the CT600 should use whole pounds)
• A line missed in the computation (e.g., disallowable expenses not added back)
• Capital allowances entered in the computation but not in Box 690

"Total income does not equal sum of components"

Box 235 (total income) should equal the sum of trading profits, investment income, and other income. If it doesn't add up, HMRC rejects.

Fix: Go through each income box and verify they sum to the total. This often happens when you update one box but forget to update the total.

"Corporation Tax payable is incorrect"

HMRC recalculates the tax based on your figures and compares it to the amount you've entered. If they don't match, the return is rejected.

Fix: Recalculate the tax manually:

• For profits up to £50,000: tax at 19%
• For profits between £50,001 and £250,000: tax at 25% with Marginal Relief
• For profits above £250,000: tax at 25%

Remember to account for associated companies when determining which rate applies. If your company has an associated company, the thresholds are divided by the number of associated companies.

2. XML Validation Errors

These errors mean the file structure itself is broken — the XML doesn't conform to HMRC's schema.

"Invalid XML: unexpected element"

Your file contains an XML element that HMRC's system doesn't recognise.

Fix: This usually indicates a software bug or outdated template. If you're using filing software, update to the latest version. If you're generating XML manually (not recommended), check against HMRC's published schema.

"Character encoding error"

Special characters (£, é, —) in company names, addresses, or notes can cause encoding issues if the file isn't UTF-8.

Fix: Ensure your filing software outputs UTF-8 encoded files. If copying text from Word or PDF, special characters like "smart quotes" or em-dashes can cause problems.

"Schema version not supported"

You're using an outdated CT600 schema version.

Fix: HMRC updates the CT600 schema periodically. Make sure your software is using the current schema version. As of 2025, the current version is CT600 (2024) Version 3.

3. iXBRL Attachment Errors

"Accounts attachment missing or invalid"

HMRC requires iXBRL accounts to be attached to the CT600. If the attachment is missing, corrupted, or not valid iXBRL, the whole submission fails.

Fix:

• Verify the accounts file opens correctly in a browser (it should display as readable HTML)
• Check the file extension is .html or .xhtml
• Ensure the file contains valid iXBRL tags (not just plain HTML)
• Verify the taxonomy references are current

"Computation attachment missing"

The iXBRL tax computation is also mandatory. Some software generates the accounts but forgets the computation.

Fix: Generate and attach both the accounts AND the tax computation. They are separate documents.

"Company registration number mismatch"

The company number in your iXBRL accounts doesn't match the one on the CT600 or HMRC's records.

Fix: Check the company registration number is identical everywhere:

• CT600 (Box 1)
• iXBRL accounts
• iXBRL computation
• Must match Companies House (including leading zeros — "01234567" not "1234567")

4. Authentication and Authorisation Errors

"Agent not authorised for this company"

If you're filing as an agent, you need HMRC authorisation for that specific company.

Fix: Log in to your Government Gateway agent account and check that the company is listed under your authorised clients. If not, you'll need the company to grant authorisation (64-8 form or online authorisation).

"UTR not recognised"

The Unique Taxpayer Reference on the CT600 doesn't match HMRC's records.

Fix: Double-check the 10-digit UTR. It's easy to transpose digits. You can find it on any previous HMRC correspondence, or call HMRC's Corporation Tax helpline.

"Government Gateway credentials invalid"

Your login details are wrong, or the account has been locked.

Fix: Reset your Government Gateway password. If the account is locked, you'll need to call HMRC or wait for it to unlock (usually 2 hours after too many failed attempts).

How to Read HMRC Error Messages

HMRC error responses come in a structured format. Here's how to decode them:

Fatal errors must be fixed before resubmission. Warnings are informational — HMRC might accept the filing but flag it for review.

If your filing software shows the raw error response, look for the <ErrorText> or <Description> elements — these contain the most useful information.

Step-by-Step: Fix and Resubmit

1. Read the error carefully — identify which boxes or attachments are causing the problem
2. Don't rush — a hasty resubmission with the same errors wastes time
3. Fix the root cause — don't just change the number that failed; understand why it was wrong
4. Cross-check all totals — if one number was wrong, others that depend on it might be too
5. Validate before resubmitting — most filing software has a "check" or "validate" function
6. Resubmit — HMRC treats this as a fresh submission; the previous rejected one doesn't exist
7. Save the confirmation — when accepted, keep the HMRC acknowledgement (IRmark and timestamp)

Does a Rejection Count as a Late Filing?

No — but the deadline doesn't pause. A rejected CT600 is treated as if it was never filed. If your deadline is 1 April and you submit on 31 March but it's rejected, you need to fix and resubmit by 1 April. If the corrected version isn't accepted until 2 April, you've filed late and penalties apply.

Practical tip: Don't leave filing to the last day. Give yourself at least a week's buffer for potential rejections. Check your exact CT600 deadline and work backwards from there.

Preventing Rejections in the First Place

The best fix is not getting rejected at all. Here's how:

Use Software That Validates Before Submission

Good CT600 filing software runs HMRC's Business Validation Rules locally before you submit. This catches 90%+ of rejection errors before they happen. Taxpipe does this automatically — it won't let you submit a return that will fail.

Double-Check These Common Trouble Spots

Before every submission, verify:

• Accounting period dates match on CT600, accounts, and computation
• Company registration number includes leading zeros
• UTR is correct (10 digits, no spaces)
• Total income equals the sum of its components
• Corporation Tax calculation uses the correct rate and Marginal Relief
• Both iXBRL attachments (accounts + computation) are included
• All mandatory iXBRL tags are present

File a Test Submission First

HMRC provides a test environment where you can submit a return and check for errors without it counting as a real filing. Your software may offer a "test" or "dry run" option — use it.

What If You Keep Getting Rejected?

If you've tried multiple times and can't resolve the error:

1. Call HMRC's Corporation Tax helpline (0300 200 3410) — they can sometimes identify the specific issue
2. Check HMRC's service availability — occasionally the Gateway has known issues (check status)
3. Try a different filing software — if the problem is with your software's XML generation, switching tools can resolve it
4. Contact your software provider — they should be able to help with technical filing errors

Related Articles

• CT600 Common Mistakes: How to Avoid Them
• CT600 Filing Checklist: What You Need
• CT600 Amendments: How to Correct Mistakes
• iXBRL Accounts: What Are They and Why You Need Them

Avoid Rejections Entirely — File With Taxpipe

Taxpipe runs comprehensive validation checks before every submission — including all HMRC Business Validation Rules, XML schema checks, and iXBRL tag verification. If something's wrong, we tell you exactly what to fix before you submit, so you never get that dreaded rejection email. Complete CT600 filing from just £59 + VAT. Start filing now →