Guide · February 2026
Filing Your First CT600: A New Company Guide
You have just incorporated a UK limited company. Congratulations. Now HMRC expects you to register for corporation tax, keep accounting records, and file a CT600 return. This guide walks you through every step, from incorporation to submission.
Step 1: Register for corporation tax
Within 3 months of your company starting to trade (or receiving any taxable income), you must register with HMRC for corporation tax. You can do this online at gov.uk.
After registering, HMRC will send your company a Unique Taxpayer Reference (UTR) by post. This typically takes 2 to 4 weeks. You need this UTR for everything that follows.
Step 2: Set up Government Gateway
To file your CT600 electronically (which is mandatory), you need a Government Gateway account linked to your company. Here is the process:
- 1Create a Government Gateway account at gov.uk (choose Organisation, then Corporation Tax)
- 2Enter your company UTR and company registration number
- 3HMRC will post an activation code to your registered office address (allow up to 2 weeks)
- 4Log back in and enter the activation code to complete enrolment
Start early. The Gateway activation process can take 2-4 weeks total. Do not wait until your filing deadline is close.
Step 3: Understand your accounting period
Your company has an accounting reference date (ARD) — the date its financial year ends. By default, Companies House sets this to the last day of the month in which you incorporated.
For corporation tax purposes, HMRC does not allow an accounting period longer than 12 months. If your first Companies House accounting period is longer (new companies can have up to 18 months for their first accounts), you must split it into two CT600 returns.
Example: Company incorporated 15 June 2025 with ARD of 30 June. First Companies House accounts cover 15 June 2025 to 30 June 2026 (12.5 months). You file two CT600s: one for 15 June 2025 to 14 June 2026 (12 months) and one for 15 June 2026 to 30 June 2026 (16 days).
Step 4: Prepare your figures
Your CT600 is based on your company accounts. At minimum, you need:
- •Profit and loss account — total income, cost of sales, expenses, and net profit
- •Balance sheet — assets, liabilities, and equity at the period end
- •Tax computation — adjustments from accounting profit to taxable profit (disallowable expenses, capital allowances)
For micro-entities using FRS 105, the accounts are simplified. See our micro-entity accounts guide for details.
Step 5: File your CT600
You cannot file a CT600 on paper. It must be submitted electronically, either through HMRC's own service (closing April 2026) or through commercial software.
Your CT600 submission includes:
- •The CT600 form itself (income, deductions, tax calculation)
- •Company accounts in iXBRL format (tagged using the appropriate taxonomy)
- •Tax computation in iXBRL format
Key deadlines
| What | When |
|---|---|
| Register for corporation tax | Within 3 months of starting to trade |
| Pay corporation tax | 9 months and 1 day after the accounting period ends |
| File CT600 return | 12 months after the accounting period ends |
| File accounts at Companies House | 9 months after the accounting period ends (21 months for first accounts) |
Late filing penalties: £100 if up to 3 months late, £200 if 3-6 months late, plus HMRC estimates your tax and charges 10% of that estimate for 6-12 months late. After 12 months, an additional 20% penalty. Interest runs from the payment due date.
Common mistakes for first-time filers
- ✗Starting Gateway enrolment too late — the activation code arrives by post and can take weeks. Start the process as soon as you have your UTR.
- ✗Confusing accounting periods — if your first period is longer than 12 months, you need two CT600 returns. Filing one return for the full period will be rejected.
- ✗Forgetting to pay before filing — corporation tax payment is due 3 months before the filing deadline. Interest starts from the payment date, not the filing date.
- ✗Not claiming capital allowances — new companies often buy equipment, computers, or furniture. These are claimed through capital allowances (up to £1M under the Annual Investment Allowance), not as direct expenses.
File your first CT600 with Taxentis
Taxentis walks you through your first CT600 step by step. Enter your company details, profit and loss figures, and balance sheet — we generate the iXBRL accounts and submit directly to HMRC.