CT603 Form – Complete Guide to Corporation Tax Return Notices in the UK
Understand Your Company’s CT603 Notice and What It Means
If you run a limited company in the United Kingdom, you may have received a letter or notice from HMRC called a CT603 Form. Many directors and business owners find this confusing — but don’t worry. It’s not a bill, and it’s not a fine. It’s a Corporation Tax Return Notice that tells you your company is required to file its annual Corporation Tax return (known as the CT600).
This guide from Audit Consulting Group explains everything you need to know about the CT603 form — what it is, why you’ve received it, what to do next, the deadlines, and how to stay compliant.
We’ll also walk you through step-by-step how to respond correctly and avoid penalties from HMRC.
What Is the CT603 Form?
The CT603 form (also called a “Notice to Deliver a Company Tax Return”) is an official notice issued by HM Revenue & Customs (HMRC).
It informs a company that it must:
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File a Company Tax Return (CT600) for a specific accounting period, and
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Pay any Corporation Tax that may be due.
Receiving a CT603 form means HMRC expects your company to submit a Corporation Tax return, even if your company made no profit or no trading activity during that period.
If you don’t respond, HMRC can charge automatic late filing penalties — even if your company owes no tax.
Why You Received a CT603 Form
HMRC issues CT603 notices when:
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You’ve registered a limited company with Companies House.
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Your company’s financial year (accounting period) has ended.
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You’ve previously submitted a Corporation Tax return, and HMRC expects another one.
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HMRC’s system indicates business activity or income.
In simple terms — if your company exists, HMRC assumes you need to report.
If you believe the company didn’t trade or was dormant, you still need to inform HMRC properly (explained below).
What the CT603 Notice Includes
The CT603 typically contains:
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Your Company Name and UTR (Unique Taxpayer Reference)
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The accounting period you need to report for
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The deadline for submitting your CT600 return
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Information on how to file online
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Contact details for HMRC
It’s usually sent to your company’s registered office address or your accountant.
What to Do After Receiving a CT603 Form – Step by Step
Step 1: Read the Notice Carefully
Check the accounting period HMRC has specified. This is often the 12-month period ending on your company’s financial year-end date.
Step 2: Confirm Your Company’s Trading Status
You must determine whether your company was:
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Active (trading) – engaged in business activities and generating income.
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Dormant – not trading or receiving any income during the period.
If dormant, you may not need to submit a full CT600, but you must inform HMRC officially.
More info: https://www.gov.uk/tell-hmrc-your-company-is-dormant-for-corporation-tax
Step 3: Prepare Your Accounts
If your company traded, prepare your statutory accounts for the same period:
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Profit and loss statement
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Balance sheet
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Notes to accounts
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Director’s report (if applicable)
These accounts must match your Corporation Tax return.
Step 4: Complete and File the CT600 Return
Your CT600 includes:
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Company details (name, UTR, accounting period)
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Income, turnover, and profit before tax
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Deductions and reliefs (like R&D or capital allowances)
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Corporation Tax due
You must file this return online using HMRC-approved software or through an accountant.
File online here: https://www.gov.uk/company-tax-return/how-to-file
Step 5: Pay Any Corporation Tax Due
Corporation Tax payment is due nine months and one day after the end of the accounting period.
Example: If your year ends 31 December 2024, payment is due by 1 October 2025.
More info: https://www.gov.uk/pay-corporation-tax
Step 6: Keep Records
HMRC requires you to retain business and accounting records for at least six years.
This includes invoices, receipts, payroll, bank statements, and electronic accounting data.
What If Your Company Is Dormant?
If your company didn’t trade or receive any income:
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You must inform HMRC via the online form:
https://www.gov.uk/tell-hmrc-your-company-is-dormant-for-corporation-tax -
HMRC will update your record and may not require a CT600 return.
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You may still need to file dormant accounts with Companies House.
Deadlines for Filing
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Corporation Tax payment – within 9 months and 1 day after year-end.
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CT600 return filing – within 12 months after year-end.
Example:
If your accounting year ends 31 March 2024:
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Payment due: 1 January 2025
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CT600 filing due: 31 March 2025
Even if your company has no tax to pay, the return must be submitted on time.
Penalties for Late Filing
HMRC automatically applies penalties if you miss your CT600 deadline:
| Delay | Penalty |
|---|---|
| 1 day late | £100 |
| 3 months late | Additional £100 |
| 6 months late | 10% of Corporation Tax due |
| 12 months late | Additional 10% of Corporation Tax due |
Repeated late submissions within 12 months increase the fines to £500 each time.
If you repeatedly ignore CT603 notices, HMRC can estimate your company’s profits and issue a tax demand — often higher than what’s actually owed.
How to Avoid Penalties
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File your return as early as possible.
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Keep your accounting software and Companies House deadlines aligned.
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Use HMRC-recognised filing software.
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Set reminders for both payment and filing deadlines.
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If you’ve stopped trading, notify HMRC to avoid future CT603 notices.
Audit Consulting Group helps companies stay compliant and avoid unnecessary fines by managing all deadlines and communications with HMRC on your behalf.
CT603 for New Companies
When you register a new limited company, HMRC automatically creates a Corporation Tax record and issues a CT603 notice to your registered address — usually within the first few weeks.
You must:
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Register your company for Corporation Tax within three months of starting to trade.
https://www.gov.uk/register-for-corporation-tax -
File your first CT600 after your first accounting period ends.
If you don’t start trading immediately, tell HMRC your company is dormant until it becomes active.
CT603 for Dormant or Closed Companies
If your company has stopped trading, you must still deal with the CT603 notice properly:
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If dormant, file a “nil” return or notify HMRC online.
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If closing the company, settle final Corporation Tax and close the record.
When closing a company officially, inform both Companies House and HMRC:
https://www.gov.uk/close-limited-company
Failure to do this can trigger continued CT603 notices and late filing penalties even after your business has stopped trading.
Changing Your Accounting Period
If the accounting period in your CT603 notice doesn’t match your actual trading year, you can request a change by writing to HMRC or updating it online through your tax account.
This is common if your company changed its year-end at Companies House.
Official info: https://www.gov.uk/corporation-tax-accounting-period
What If You Disagree with the CT603 Notice?
If you believe HMRC issued the notice in error (for example, your company was dormant or not yet trading):
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Contact HMRC’s Corporation Tax helpline immediately.
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Provide evidence of your company’s inactivity.
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Request that the notice be withdrawn or marked dormant.
Corporation Tax helpline: 0300 200 3410
Always keep written confirmation or email evidence from HMRC.
How to File the CT600 (Response to CT603)
You can file online using:
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HMRC’s free online service for small companies, or
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Commercial accounting software (e.g. Taxfiler, Sage, QuickBooks, Xero, FreeAgent).
Most companies prefer their accountant to handle CT600 filing to ensure accuracy and correct tax calculations.
Audit Consulting Group uses HMRC-approved software and reviews every figure to ensure the return is correct and compliant.
How Audit Consulting Group Can Help
Responding to a CT603 notice can be stressful, especially for first-time directors or small business owners.
Our experienced accountants and tax consultants can:
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Review your CT603 notice and confirm what HMRC requires.
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Prepare and file your Corporation Tax return (CT600).
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Ensure correct alignment with Companies House accounts.
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Handle HMRC communication and queries.
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Advise on reliefs such as R&D, capital allowances, or losses carried forward.
We help you avoid penalties, maximise tax efficiency, and keep your company in good standing with HMRC.
Case Studies
Case 1 – First-Time Business Owner
Client: Emma R., eCommerce Retailer
Issue: Received CT603 notice after registering her business but didn’t realise she had to file a CT600.
Solution: We prepared her first accounts, submitted CT600, and avoided a potential £100 fine.
Case 2 – Dormant Company Misunderstanding
Client: S. Hughes, Consulting Firm
Issue: Company went dormant but kept receiving CT603 notices.
Solution: We informed HMRC of dormant status, stopped future notices, and ensured no penalties applied.
Case 3 – Late Filing Penalty Appeal
Client: GreenTech Systems Ltd
Issue: Missed CT600 deadline by 5 days due to software issue.
Solution: We filed an appeal with supporting evidence; HMRC cancelled £100 penalty.
Case 4 – Corporation Tax Refund
Client: Luma Digital Ltd
Issue: Overpaid Corporation Tax after amending accounts.
Solution: Audit Consulting Group submitted an amended CT600; refund of £2,450 issued within six weeks.
Frequently Asked Questions (FAQ)
Q1: What exactly is the CT603?
It’s a notice from HMRC requiring your company to deliver a Corporation Tax return for a specific accounting period.
Q2: Do I need to respond if my company made no profit?
Yes. Even with no profit, you must file a “nil” return or inform HMRC that your company is dormant.
Q3: What happens if I ignore it?
HMRC will issue penalties and may estimate your tax, often overestimating what’s owed.
Q4: Can I submit the CT600 myself?
Yes, but using an accountant ensures accuracy and avoids penalties.
Q5: How do I pay Corporation Tax?
Online via bank transfer, debit card, or corporate tax account — details at: https://www.gov.uk/pay-corporation-tax
Q6: Can I get an extension?
Only in exceptional circumstances. Contact HMRC before the deadline.
Q7: What if I dissolved my company?
You must still submit a final return and settle tax before closing. Then HMRC will stop sending CT603 notices.
Common Mistakes to Avoid
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Ignoring the CT603 notice thinking it’s “just a letter.”
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Filing late or submitting incomplete accounts.
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Using the wrong accounting period dates.
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Failing to match Companies House and HMRC filings.
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Not telling HMRC when your company becomes dormant.
Each of these can lead to unnecessary penalties or compliance issues.
When to Seek Professional Help
If you’re unsure how to handle a CT603, or if you’ve already received late filing penalties, professional help can save time and stress.
Audit Consulting Group offers comprehensive Corporation Tax compliance services — from first-time filings to penalty appeals and refunds.
We’ll handle the entire process for you, ensuring that your accounts and tax returns are filed correctly, on time, and in full compliance with UK law.
Start Your CT603 Response Today
Don’t ignore your CT603 notice — let our team handle it quickly and correctly.
We’ll explain what HMRC needs, prepare your CT600, calculate any Corporation Tax due or refunds, and submit everything securely.
Your peace of mind and compliance are our priority.
Official guidance:
https://www.gov.uk/company-tax-returns
