Filing accounts electronically needs the company’s authentication code: a six-character code, made of letters and numbers, that Companies House issues to each company. It works like a signature for online filings, so Companies House can tell the filing is authorised. Tax Optimiser can’t submit your accounts without it.
Where to find it
Companies House posts the code to the company’s registered office address when the company is incorporated or when someone asks for one. If you don’t have it:
- check with the company’s directors, secretary or former accountant, who may have a copy;
- request it from Companies House online. The letter goes to the registered office and can take up to five working days, so ask early if a deadline is close.
Keep it private. Anyone with the code can file on the company’s behalf. Only share it with people you trust to file for the company, such as your accountant, and ask Companies House for a new one if you think it has been misused.
Entering it in Tax Optimiser
Open the accounting period. The Setup Process Checklist shows an Authentication Code line:
- Companies House authentication code required — click Enter Code, type the six characters and click Save.
- Companies House authentication code is set — the code is saved. Click Update if it has changed.
You can also store it on the organisation settings page in the Companies House Authentication Code field, so each new period picks it up. The Ready to Submit step only appears once the code, the presenter credentials and the validation checks are all in place.
Accountants filing for clients also need a Companies House presenter ID — see Firm settings for filing.
If a filing is rejected for the code
A rejection that mentions the authentication code usually means the code was mistyped or has been replaced by a newer one. Check it against the latest letter from Companies House, update it with Update, and submit again. Remember the filing deadline still applies — see late filing penalties.
What is a Companies House authentication code?
A six-character code of letters and numbers that Companies House issues to each company. It is needed to file information, including accounts, online or through software.
How do I get my company authentication code?
It is posted to the registered office address. If you have lost it, request it from Companies House online; the new letter can take up to five working days to arrive.
Where do I enter the authentication code in Tax Optimiser?
On the accounting period’s Setup Process Checklist, click Enter Code (or Update) next to Authentication Code. You can also save it in the Companies House Authentication Code field of the organisation settings.
Is the authentication code the same as the UTR?
No. The UTR is HMRC’s ten-digit Corporation Tax reference. The authentication code is Companies House’s six-character filing code.