10 CT600 Mistakes That Get Your Return Rejected by HMRC
Filing your CT600 wrong doesn't just delay things — it can mean penalties, interest charges, and having to refile. Here are the mistakes HMRC sees most often.
1. Wrong UTR
The number one cause of instant rejection. Your UTR is 10 digits, not 8 (that's your company number). Double-check it on your HMRC correspondence.
2. Period Dates Don't Match
Your CT600 accounting period must match what HMRC expects. If you changed your year-end with Companies House but didn't tell HMRC, your return will be rejected.
Common issues:
- Period overlaps with a previous return
- Period is longer than 12 months (split into two returns)
- Dates don't match the CT603 notice
3. Missing Mandatory Boxes
Certain boxes must be completed even if the value is zero:
- Box 145 (turnover) — enter 0, not blank
- Box 315 (total profits)
- Box 525 (tax payable)
- Box 80 (declaration)
4. Accounts Period ≠ CT600 Period
Your company accounts filed at Companies House can cover up to 18 months. But a CT600 can only cover up to 12 months. If your accounts period is longer, you need two CT600s.
5. Incorrect Company Type
Box 4 (company type code) must match your company's actual legal status. Most private limited companies are type "00". Getting this wrong causes validation errors.
6. Mathematical Errors
HMRC's system cross-checks your figures. If Box 315 (total profits) doesn't equal the sum of its components, or if the tax calculation doesn't match the rates, your return will fail validation.
Taxpipe calculates all boxes automatically, eliminating math errors.
7. iXBRL Format Errors
Your accounts and tax computations must be in valid iXBRL format. Common issues:
- Using the wrong taxonomy year
- Missing required XBRL tags
- Schema validation errors
This is one reason to use software rather than trying to file manually.
8. Stale Company Data
If your company name, registered office, or other details changed since your last filing but you haven't updated Companies House, there can be mismatches.
9. Filing for a Dissolved Company
If your company was struck off the register before you filed, HMRC can't accept the return. You may need to restore the company first.
10. Duplicate Submission
Submitting the same return twice (e.g., because the first seemed to hang) can cause confusion. HMRC's system deduplicates, but it can delay processing.
How to Avoid Rejections
- Use validated software — Taxpipe validates your return against HMRC's rules before submission
- Check your UTR — verify against an HMRC letter
- Match your dates — use the exact dates from your CT603 notice
- File early — gives you time to fix issues before the deadline
File your CT600 with Taxpipe — we validate everything before submission. £59, no hidden fees.