How to File CT600 Online in 2026: Complete Step-by-Step Guide

If you're searching for how to file your CT600 online in 2026, you've probably just discovered that HMRC's free Corporation Tax Online filing service is closing on 31 March 2026.

Don't panic. Filing your CT600 online is still straightforward — you just need to use commercial software instead of HMRC's website. This guide walks you through the entire process, step by step.

What's Changed: Why You Can't File Through HMRC Anymore

Until March 2026, HMRC offered a free online portal where company directors could file CT600 returns directly. That service is being retired.

From 1 April 2026, every UK limited company must use HMRC-recognised commercial software to file their corporation tax return. There are no exceptions — even dormant companies need to file through software.

HMRC's reasoning is that the free service was built on ageing technology and is expensive to maintain. They believe the commercial software market now offers better, more reliable options.

The bottom line: you need software. The good news is it doesn't have to be expensive or complicated.

What You Need Before You Start

Before you sit down to file your CT600, gather these documents and information:

Company Details

Company Registration Number (CRN) — your 8-digit Companies House number
Corporation Tax UTR — your 10-digit Unique Taxpayer Reference from HMRC
Accounting period dates — the start and end dates of the period you're filing for

Financial Information

Turnover — total sales/revenue for the period
Cost of sales — direct costs of goods or services sold
Operating expenses — rent, utilities, salaries, software, travel, etc.
Bank interest received — even if it's just a few pounds
Any capital allowances — equipment, vehicles, or other qualifying assets
Dividends paid — amounts and dates of any dividends declared

Previous Return Details

Losses brought forward (if any) from previous periods
Capital allowance pool values from your last return

Tip: If this is your company's first CT600, you won't have previous return details. Most fields will be zero or blank, which makes your first filing simpler.

Step 1: Choose Your Filing Software

You need HMRC-recognised software that can submit CT600 returns electronically. Here's what to look for:

HMRC-recognised — the software must be on HMRC's approved list
iXBRL tagging — your accounts must be submitted in iXBRL format (the software handles this)
CT600 computation — ideally, the software should calculate your tax, not just let you fill in boxes
Reasonable pricing — you shouldn't need to pay hundreds of pounds

How the Options Compare

Software	Price	What You Get
Taxpipe	£59 one-off	CT600 + iXBRL accounts, guided workflow, instant HMRC submission
TaxCalc	£136.80/year	Annual subscription, desktop software
Andica	£85+/year	Subscription, more complex interface
GoSimpleTax	£90+	Corporation tax as an add-on module
Full accounting platforms	£20-40/month	Xero, FreeAgent — CT600 included but requires full subscription

Taxpipe is built specifically for this situation — company directors who need to file a CT600 without paying for software they don't need. At £59 per filing, it's the most affordable dedicated option.

Step 2: Set Up Your Account

Using Taxpipe as our example (since it's the simplest process):

Go to taxpipe.co.uk/signup and create your account
Enter your company details — CRN, UTR, and registered address
Confirm your accounting period — the dates your return covers

The software will verify your company details against Companies House automatically. This takes seconds and ensures everything matches HMRC's records.

Step 3: Enter Your Financial Information

This is where you input your company's numbers. If you've kept reasonable records throughout the year, this should take 15-30 minutes.

Turnover and Costs

Enter your:

Total turnover (sales revenue)
Cost of sales (direct costs)
Gross profit (the software calculates this)
Administrative expenses (overheads, salaries, etc.)
Other operating income (if any)

Tax Adjustments

Some expenses in your accounts aren't deductible for tax purposes. Common adjustments include:

Entertaining — client entertainment is never deductible
Depreciation — replaced by capital allowances for tax purposes
Fines and penalties — not deductible
Personal expenses put through the company — must be added back

Taxpipe flags common adjustments automatically, so you're less likely to miss something or claim something you shouldn't.

Capital Allowances

If your company has purchased equipment, vehicles, or other qualifying assets, you can claim capital allowances instead of depreciation:

Annual Investment Allowance (AIA): 100% deduction on the first £1,000,000 of qualifying expenditure
Full Expensing: 100% first-year allowance on qualifying plant and machinery (permanent from April 2023)
Writing Down Allowance: 18% or 6% per year on the reducing balance for assets not fully expensed

Enter your capital expenditure, and the software calculates the allowances. Not sure what qualifies? Use our corporation tax calculator to estimate your position before you start.

Bank Interest and Other Income

Don't forget to include:

Bank interest received (even small amounts)
Rental income (if your company lets property)
Investment income
Any other non-trading income

Step 4: Review Your Corporation Tax Calculation

Once you've entered everything, the software computes your corporation tax liability.

Understanding the Tax Rates (2025/26)

Your tax rate depends on your company's taxable profits:

Profit Band	Tax Rate	Notes
£0 - £50,000	19%	Small profits rate
£50,001 - £250,000	26.5% effective	Marginal relief applies
Over £250,000	25%	Main rate

Important: If your company has associated companies (companies controlled by the same people), the profit thresholds are divided by the number of associated companies. Two associated companies means the small profits threshold drops to £25,000.

The software handles marginal relief calculations automatically. If your profits are between £50,000 and £250,000, the effective rate will be somewhere between 19% and 25%.

Check the Numbers

Before submitting, review:

✅ Turnover matches your records
✅ Expenses look reasonable
✅ Capital allowances are correct
✅ The tax liability makes sense given your profit level
✅ Your accounting period dates are right

Step 5: Generate iXBRL Accounts

Your CT600 must be accompanied by accounts in iXBRL format (Inline eXtensible Business Reporting Language). This is a tagged data format that lets HMRC's computers read your accounts automatically.

You don't need to understand iXBRL — the software generates it for you. But you do need to ensure:

Your accounts are micro-entity or small company format (whichever applies)
The accounts cover the same period as your CT600
Director details and registered address are correct

Taxpipe generates compliant iXBRL accounts automatically from the financial information you've entered. No separate accounts preparation needed.

Step 6: Submit to HMRC

This is the moment of truth. When you click submit:

The software sends your CT600 and iXBRL accounts to HMRC electronically
HMRC's system validates the submission
You receive a confirmation with an IRmark (a digital receipt)
HMRC processes your return (usually within 24-72 hours)

Save your IRmark. This is your proof of filing. HMRC sends a formal acknowledgement, but the IRmark is your immediate confirmation.

Step 7: Pay Your Corporation Tax

Filing the CT600 and paying the tax are separate processes. After filing, you need to pay any tax owed.

Payment Deadline

Corporation tax is due 9 months and 1 day after the end of your accounting period. For example:

Accounting period ends 31 March 2026 → Tax due by 1 January 2027
Accounting period ends 31 December 2025 → Tax due by 1 October 2026

Note: The filing deadline is 12 months after your accounting period ends, but the payment deadline is earlier. Many directors confuse these dates.

How to Pay

You can pay HMRC via:

Online banking (Faster Payments) — reference is your UTR + accounting period
Direct Debit — set up through your HMRC online account
BACS — bank transfer
Corporate credit card — HMRC charges a fee for this

Never miss the payment deadline. HMRC charges interest from the day after the due date, plus penalties for late payment.

Common Mistakes to Avoid

1. Wrong Accounting Period Dates

Your CT600 must match your company's accounting period exactly. If your accounts run to 31 March but you enter 30 March, HMRC will reject the filing.

2. Forgetting Bank Interest

Even £2.37 of bank interest needs to be declared. HMRC cross-references with banks.

3. Claiming Disallowable Expenses

Client entertaining, personal expenses, and non-business costs must be added back. Claiming them triggers enquiries.

4. Missing the Filing Deadline

The CT600 is due 12 months after your accounting period ends. Miss it and you face:

£100 penalty immediately
Another £100 after 3 months
10% of unpaid tax after 6 months
A further 10% after 12 months

5. Not Filing for Dormant Companies

Even if your company did nothing all year, you must file a CT600. Dormant companies still need to file — the return is simpler, but it's not optional.

What If My Company Is Dormant?

If your company had no transactions during the period (truly dormant), your CT600 is much simpler:

Turnover: £0
Expenses: £0
Tax liability: £0

Taxpipe has a dedicated dormant company shortcut that pre-fills everything and lets you file a dormant CT600 in under 5 minutes. It's the fastest way to stay compliant without paying for a full accounting service.

Frequently Asked Questions

Can I still file through HMRC's website?

Not after 31 March 2026. The free service is closing permanently. You must use commercial software.

How much does CT600 software cost?

From £59 (Taxpipe) to several hundred pounds per year. Compare pricing here.

Do I need an accountant?

No. If your company's affairs are straightforward (most small companies), you can file the CT600 yourself using software like Taxpipe. An accountant typically charges £300-800+ for the same filing.

What if I've never filed a CT600 before?

The process is the same — you just won't have any brought-forward figures. Your first filing is often the simplest.

Can I file a CT600 for previous years?

Yes. You can file CT600 returns for any open period. If you're late, you'll face penalties, but it's better to file late than not at all.

What's the difference between CT600 and annual accounts?

The CT600 is your corporation tax return, filed with HMRC. Annual accounts are filed separately with Companies House. They're different obligations with different deadlines, though the financial figures overlap.

With Taxpipe, your CT600 submission includes the required iXBRL-tagged accounts, so you're covering the HMRC side completely.

Summary: File Your CT600 in 2026

Here's the process in brief:

Choose software — Taxpipe at £59 is the simplest, most affordable option
Enter your financials — turnover, expenses, capital allowances
Review the calculation — check the tax figure makes sense
Submit electronically — get your IRmark confirmation
Pay on time — 9 months and 1 day after your period end

The transition from HMRC's free service to commercial software might feel like an annoyance, but it's genuinely straightforward. Tools like Taxpipe exist specifically to make this painless.

Start your CT600 filing now →

This guide was last updated in February 2026. Corporation tax rates and thresholds are for the 2025/26 tax year. Always check HMRC's website for the latest information.