How to File a CT600 Online — Step by Step Guide

If you run a limited company in the UK, you need to file a CT600 corporation tax return with HMRC every year — even if your company made no profit. It sounds intimidating, but the process is more straightforward than most directors expect.

This guide walks you through how to file a CT600 online, step by step, in plain English. No jargon, no accounting degree required.

What Is a CT600?

The CT600 is the form your company uses to tell HMRC how much corporation tax it owes. It reports your company's taxable profits (or losses), capital allowances, and any reliefs you're claiming for a specific accounting period.

Every UK limited company registered with HMRC for Corporation Tax must file one. That includes dormant companies — if the company exists, HMRC expects a return.

When Is the CT600 Due?

You must file your CT600 within 12 months of the end of your accounting period. For most companies, the accounting period matches the financial year shown at Companies House.

For example, if your accounting period ends on 31 March 2026, your CT600 is due by 31 March 2027.

However, the corporation tax payment deadline is earlier — you must pay any tax owed within 9 months and 1 day of the accounting period end. Using the same example, payment would be due by 1 January 2027.

Missing either deadline means penalties and interest, so mark both dates in your calendar.

Late Filing Penalties

How late	Penalty
1 day	£100
3 months	Another £100
6 months	HMRC estimates your tax and adds 10%
12 months	Another 10% of unpaid tax

These stack up quickly, and HMRC charges daily interest on top.

What You Need Before You Start

Before you sit down to file your CT600 online, gather the following:

1. Company Accounts

You need your company's statutory accounts for the period. These include your profit and loss statement (or income statement) and balance sheet. If you use accounting software like Xero, FreeAgent, or QuickBooks, you can export these.

2. Your Company's UTR

Your Unique Taxpayer Reference (UTR) is a 10-digit number HMRC assigned when your company registered for Corporation Tax. You'll find it on letters from HMRC or in your HMRC online account.

3. HMRC Online Account or Agent Credentials

To file online, you need either your company's Government Gateway credentials or access through an accountant's agent portal.

4. Computations (Tax Calculation)

Your corporation tax computation adjusts your accounting profit into taxable profit. This means adding back disallowable expenses (like entertaining clients or certain depreciation) and deducting capital allowances and reliefs.

5. iXBRL Accounts

Here's the part that trips most people up. HMRC requires your accounts to be filed in iXBRL format (inline eXtensible Business Reporting Language). You can't just upload a PDF. Your accounts must be tagged with standardised codes so HMRC's systems can read them.

This is where most DIY filers hit a wall — and where tools like Taxpipe come in handy, as they handle the iXBRL tagging and conversion automatically.

How to File a CT600 Online — Step by Step

Step 1: Log In to HMRC's Online Service (or Use Commercial Software)

HMRC retired its own CT600 filing portal in April 2024. You now need commercial software to file your CT600. HMRC maintains a list of approved software providers on GOV.UK.

Some options are full-blown accounting suites. Others, like Taxpipe, focus specifically on making CT600 filing simple for directors who already have their numbers ready.

Step 2: Enter Your Company Details

Start by entering the basics:

Company name and UTR
Accounting period start and end dates
Company type (most small businesses are "trading" or "investment" companies)

Double-check the accounting period dates match exactly what HMRC expects. If your company has a non-standard year end or has changed its accounting reference date, these dates must align with HMRC's records.

Step 3: Complete the CT600 Core Sections

The CT600 has several sections. Here's what each one covers:

Box 1–35: About Your Company

Basic company information — trading status, whether you're part of a group, and your registered office address.

Box 40–55: Tax Calculation (Turnover and Income)

This is the heart of the return:

Box 40 — Turnover: Your total sales/revenue for the period
Box 55 — Trading profits: Your adjusted taxable trading profit after adding back disallowable expenses and deducting capital allowances

If you're a small company with straightforward income, these are often the most important boxes.

Box 60–90: Deductions and Reliefs

Claim any reliefs here, including:

Trading losses carried forward or back
R&D tax relief (if applicable)
Management expenses (for investment companies)
Charitable donations (Gift Aid to qualifying charities)

Many small companies won't need most of these, but it's worth checking — unclaimed reliefs mean you could be overpaying tax.

Box 95–120: Tax Calculation

The software calculates your Corporation Tax liability based on:

Taxable total profits (box 155)
The applicable tax rate (currently 19% for profits up to £50,000, 25% for profits above £250,000, with marginal relief in between)

Box 145–150: Tax Already Paid

Record any Corporation Tax already paid, including payments on account.

Box 155–235: Summary and Declaration

Final figures and a declaration that the return is correct and complete. The person signing must be a company officer (director or company secretary).

Step 4: Prepare and Attach Your Accounts in iXBRL

Your CT600 must include:

Statutory accounts in iXBRL format
Tax computations in iXBRL format

As mentioned earlier, iXBRL is mandatory. The accounts need to be tagged against the UK GAAP or FRS 102 taxonomy (or FRS 105 for micro-entities).

If you're using Taxpipe, you enter your figures and it generates the iXBRL-tagged accounts and computations for you — no need to understand the tagging yourself.

Step 5: Review Everything

Before submitting, review:

Are the accounting period dates correct?
Does the turnover match your accounts?
Have you included all income sources?
Are disallowable expenses added back?
Have you claimed all available reliefs and capital allowances?
Do the accounts figures tie back to your bookkeeping?

A few minutes of checking can save months of correspondence with HMRC.

Step 6: Submit to HMRC

Hit submit. Your software sends the CT600 and iXBRL accounts to HMRC electronically. You should receive a submission confirmation (an IRMark receipt) almost immediately. Save this — it's your proof of filing.

Step 7: Pay Your Corporation Tax

Filing and paying are separate. After submitting your return, pay any tax owed via:

Direct bank transfer (CHAPS, Bacs, or Faster Payments) using HMRC's bank details and your UTR as the reference
Direct Debit (set up through your HMRC online account)
Corporate credit/debit card (through HMRC's BillPay service)

Remember, payment is due 9 months and 1 day after your accounting period ends — which is usually before the filing deadline.

Common Mistakes When Filing a CT600

1. Wrong Accounting Period Dates

This is the most common rejection reason. Your CT600 dates must match what HMRC has on file. If your company changed its year end at Companies House but didn't tell HMRC, the return will bounce.

2. Forgetting to Add Back Disallowable Expenses

Not all business expenses reduce your tax bill. Client entertaining, some legal costs, and accounting depreciation must be added back in your tax computation. Capital allowances replace depreciation for tax purposes.

3. Missing the Payment Deadline

Directors often confuse the filing deadline (12 months) with the payment deadline (9 months and 1 day). By the time they file, they may already owe late payment interest.

4. Not Filing for Dormant Companies

If your company did nothing all year, you still need to file a CT600 — just a simpler one showing nil activity. Ignoring it triggers automatic penalties.

5. Incorrect iXBRL Tagging

If accounts are tagged incorrectly or use the wrong taxonomy, HMRC will reject the submission. This is rarely an issue with established software but can happen with manual tagging.

6. Forgetting Supplementary Pages

Some companies need additional pages alongside the CT600:

CT600A — Loans to participators (if the company lent money to directors)
CT600B — Controlled foreign companies
CT600C — Group and consortium relief
CT600E — Charities and community amateur sports clubs

Most small companies only encounter CT600A — typically when a director's loan account is overdrawn at the period end.

What Happens After You File?

Once HMRC receives your CT600:

You get an IRMark receipt confirming submission
HMRC processes the return — usually within a few days
Your HMRC online account updates to show the return as filed
HMRC may open an enquiry within 12 months of the filing date (rare, but possible)

If you've overpaid tax, HMRC will issue a refund — though this can take several weeks. If you've underpaid, you'll receive a notice with the amount due plus any interest.

Keep your records for at least 6 years after the end of the accounting period. HMRC can enquire into a return for up to 6 years (or 20 years if they suspect fraud).

Can You File a CT600 Yourself?

Yes. There's no legal requirement to use an accountant. Many directors of small companies with simple affairs file their own CT600 successfully.

That said, the iXBRL requirement and tax computation adjustments are genuine hurdles. If your company has straightforward trading income, few expenses, and no complex reliefs, a tool like Taxpipe makes self-filing realistic — it walks you through the CT600 sections, generates the iXBRL accounts, and submits directly to HMRC.

If your affairs are more complex (multiple income streams, R&D claims, group structures), consider getting professional advice at least for the first year.

Quick Checklist: Filing Your CT600

Gather your company accounts, UTR, and Gateway credentials
Prepare your tax computation (adjusting profit for disallowable expenses and capital allowances)
Choose HMRC-recognised filing software
Enter company details and accounting period
Complete the CT600 boxes — turnover, profits, reliefs
Generate iXBRL-tagged accounts and computations
Review all figures carefully
Submit electronically and save your IRMark receipt
Pay any Corporation Tax owed before the 9-month deadline

File Your CT600 the Easy Way

Filing a CT600 doesn't have to mean expensive accountant fees or wrestling with iXBRL tagging. Taxpipe is built for company directors who want to file their own corporation tax return — quickly, correctly, and without the headache.

Sign up at justfile.co.uk and file your next CT600 in minutes, not hours.

Ready to file?

Taxpipe makes CT600 filing simple. Our guided wizard handles everything — tax computation, iXBRL accounts, and direct HMRC submission. One-time fee of £59, no subscription.

File your CT600 now →

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