How Do I File CT600 Online?

Every UK limited company must file a company tax return with HMRC. This form — officially called the CT600 — reports your company's profits and calculates how much corporation tax you owe. Filing is done online for almost all companies, with HMRC only accepting paper returns in exceptional circumstances.

This guide explains exactly how to file your CT600 online, what you'll need before you start, and the key deadlines that apply.

Who Must File a Company Tax Return?

Any company or unincorporated association that receives a "notice to deliver" from HMRC must submit a return. This applies even if your company:

• Made a loss for the year
• Owes no corporation tax
• Was dormant for part or all of the period

Dormant companies still need to file a CT600 in most cases, though the return will be simpler. Sole traders and partnerships are excluded — they file Self Assessment returns instead.

HMRC requires online filing for almost all companies. The only exceptions are if you have a reasonable excuse for filing on paper, or if you are filing in Welsh.

What You Need Before You Start

Gathering the right information before you begin saves time and prevents errors. You'll need:

HMRC credentials

• Your company's Unique Taxpayer Reference (UTR) — sent by HMRC when you registered for corporation tax
• Government Gateway user ID and password for your company

Financial documents

• Your final company accounts for the accounting period
• A tax computation showing how your accounting profit is adjusted to arrive at taxable profit
• Records of any capital allowances, reliefs, or credits you are claiming

Technical requirements

• Your accounts must be in iXBRL format for submission to HMRC — most CT600 software handles this automatically

If your accounts were prepared in a standard format (PDF, Excel, or Word), you'll need to convert them to iXBRL before filing. Most dedicated CT600 software does this as part of the filing process.

Step-by-Step: How to File CT600 Online

Step 1 — Calculate Your Taxable Profit

Start with your accounting profit from your company accounts. Then apply tax adjustments:

• Add back any non-allowable expenses (such as client entertainment or fines)
• Deduct allowable reliefs such as capital allowances, R&D credits, and losses carried forward
• Arrive at your taxable profit (or loss) for the period

If your accounting period is longer than 12 months, HMRC requires you to split it into two separate CT600 returns — one for the first 12 months and one for the remainder.

Step 2 — Prepare iXBRL Accounts

HMRC requires your company accounts to be submitted in iXBRL (Inline eXtensible Business Reporting Language) format. This is a machine-readable format where every financial figure is tagged with a standardised label from the HMRC taxonomy.

Dedicated CT600 software generates iXBRL accounts automatically from the figures you enter. If you are working with an accountant who provides PDF accounts, check whether they can also supply the iXBRL version.

Step 3 — Choose Your Filing Route

There are two main ways to file the CT600 online:

HMRC's online service — the government's own free filing tool. Note that HMRC's free filing service is closing, so check whether it is still available for your filing period before relying on it.

Third-party CT600 software — dedicated tools that prepare the CT600 form, generate iXBRL accounts, and submit directly to HMRC in a single workflow. This is the most common route for self-filers and accountants alike.

Step 4 — Complete the CT600 Form

The CT600 form captures your company's financial and tax information across several sections:

Section	What it covers
Company details	Name, UTR, accounting period dates
Trading income	Turnover, adjusted profits or losses
Capital allowances	AIA, writing-down allowances, special rate pool
Reliefs and credits	R&D, losses, group relief
Tax calculation	Corporation tax at applicable rates
Payments	Tax already paid, overpayments

For a detailed breakdown of every box and what goes in it, see our CT600 boxes explained guide.

Step 5 — Submit to HMRC

Once the form is complete and your iXBRL accounts are attached, submit via your chosen filing route. HMRC returns a confirmation reference number immediately. Keep this safe — it's your proof of submission.

Step 6 — File Your Accounts with Companies House

Filing with HMRC satisfies your corporation tax obligation, but you also have a separate duty to file accounts with Companies House. These have their own deadline and format requirements. See Companies House vs HMRC filing for how the two requirements differ.

Filing Deadlines

Two key dates apply to every company:

Obligation	Deadline
File the CT600 return	12 months after your accounting period ends
Pay any corporation tax owed	9 months and 1 day after your accounting period ends

The payment deadline falls before the filing deadline. You need to estimate your tax liability early enough to pay on time, even if you have not yet finalised the return. Large companies with profits above £1.5 million face quarterly instalment payments on a different schedule.

Full guidance on how deadlines work — including for first-year companies — is in our CT600 deadlines guide.

Source: GOV.UK — Company tax returns

Penalties for Late Filing

HMRC charges automatic penalties if your return is filed late, regardless of whether any tax is owed:

How late	Penalty
1 day	£200
3 months	A further £200
6 months	10% of unpaid corporation tax
12 months	A further 10% of unpaid tax
3 consecutive late returns	Fixed penalties rise to £1,000 each

Even if your company owes no tax, the £200 fixed penalties still apply. For full details, see our CT600 penalties guide.

Source: GOV.UK — Corporation Tax: late filing penalties

Can I File the CT600 Myself?

Yes — many directors file their own CT600 without an accountant. The process is manageable if your company's tax affairs are straightforward: no R&D claims, no group relief, and no complex capital allowances situations.

You'll need CT600 software capable of generating iXBRL accounts, along with enough confidence to interpret your accounting figures and apply basic tax adjustments. If your situation is more complex, or you're unsure whether you're applying reliefs correctly, professional advice may be worthwhile.

Common Mistakes When Filing Online

• Wrong accounting period dates — ensure the period in your return matches what HMRC has on record
• Forgetting iXBRL accounts — the CT600 form alone is not sufficient; accounts must be attached
• Missing the payment deadline — tax is due before the return, so plan cashflow accordingly
• Using the wrong UTR — easy to confuse if you have multiple companies
• Not filing when making a loss — you must still file even if no corporation tax is owed

Summary

Filing your CT600 online requires your HMRC credentials, iXBRL-formatted company accounts, and a completed tax return. The return is due 12 months after your accounting period ends, but any tax owed must be paid 9 months and 1 day after the period ends. Late filing triggers automatic penalties starting at £200 even when no tax is due.

For your first filing, see our guide to first company tax returns for what to expect at each stage.