How to file a dormant company CT600 online

By Max Szolcek, founder of Dormant Return. Last updated 21 August 2026

Since HMRC's free filing service closed on 31 March 2026, a Company Tax Return has to be filed through commercial software. This guide walks through what you need before you start, what actually gets filed for a dormant company, and what happens after you press file. It uses Dormant Return as the example, but the moving parts are the same whichever software you choose.

Before you start: three things to have ready

1. The company's Corporation Tax UTR. A 10-digit Unique Taxpayer Reference, printed on letters from HMRC to the company (for example the CT603 notice to deliver a return, or the letter sent shortly after incorporation). It is not public and cannot be looked up online; if it is lost, request a copy from HMRC via gov.uk.
2. A Government Gateway sign-in with Corporation Tax enrolment. This is the 12-digit user ID and password the company uses for its HMRC business tax account, and the account must have an active Corporation Tax enrolment for this company's UTR. Note this is the older Government Gateway ID, not a GOV.UK One Login email sign-in. If the company has never enrolled for Corporation Tax online, do that first at gov.uk; activation can take a few days.
3. The accounting period HMRC expects. Your business tax account lists outstanding Corporation Tax obligations with their exact start and end dates. The period on the return must match what HMRC expects, or the filing will be rejected.

Not sure the company needs to file at all? Read do dormant companies need to file a CT600? first; if HMRC has not asked for a return, you may be able to tell HMRC the company is dormant instead.

Step 1: enter the company's details

In Dormant Return, start typing the company name and pick the company from the Companies House register; the registered name and company number fill in automatically, and the tool suggests the accounting period Companies House is expecting, which for most dormant companies lines up with the HMRC period. You confirm the period rather than trusting the suggestion, because HMRC's records are the ones that count. Then add the UTR and the date the directors approved the accounts.

Step 2: review what will be filed

Two documents go to HMRC in one submission:

• The CT600 return: company details, the accounting period, and nil figures for turnover, profits and tax.
• Dormant accounts in iXBRL: a short balance sheet and the statutory declarations, tagged in the machine-readable format HMRC requires. What that means is covered in dormant company iXBRL accounts explained.

Dormant Return shows you the actual accounts document exactly as HMRC will receive it, and you can download a PDF copy. Payment is a one-off £5 through Stripe at this point; there is no account to create.

Step 3: file under your own Government Gateway sign-in

You enter the company's Government Gateway user ID and password, and the return is submitted directly to HMRC's Corporation Tax online service. Filing under your own credentials matters: the submission is legally the company's own, made by you, just as it was through HMRC's old free service. Dormant Return never stores your password.

HMRC's service validates the return in full and responds within seconds to a few minutes. You watch the progress live: submitted, acknowledged, then accepted, with HMRC's cryptographic receipt (the IRmark) recorded as evidence of exactly what was filed.

What happens after filing

• Accepted: you get an on-screen receipt and an email containing HMRC's acceptance evidence and a PDF of the filed accounts. Keep it with the company's records. The obligation disappears from the business tax account, usually within a day or two.
• Rejected: nothing was filed. The most common live rejections are a UTR or company number that does not match HMRC's records, or a duplicate return for the period. With Dormant Return the £5 is refunded automatically on any rejection; fix the detail and start again.
• Sign-in failed: if the Government Gateway refuses the user ID or password, the return is never even looked at. Check you are using the company's 12-digit Gateway ID (not One Login) and that the Corporation Tax enrolment is active, then try again.

Remember Companies House is separate

Filing the CT600 to HMRC does not file anything at Companies House. Your dormant company still owes Companies House its annual accounts (dormant companies can file simple dormant accounts for free) and a confirmation statement. The split is explained in dormant for Companies House vs dormant for Corporation Tax.

Related guides

• Enrol for Corporation Tax online before you file
• How to find your company's Corporation Tax UTR
• CT600 rejected by HMRC: what the error codes mean
• Your first Company Tax Return: why HMRC expects two CT600s
• Do dormant companies need to file a CT600?
• HMRC's free filing service has closed: your options
• Late CT600 penalties for dormant companies

Sources

• GOV.UK: Company Tax Returns
• GOV.UK: Find a lost UTR number
• GOV.UK: Closure of the service to file your company accounts and tax return