What is a Companies House authentication code?
By DormantFile · Updated 27 May 2026
A Companies House authentication code is a 6-character alphanumeric code (like A1B2C3) that proves you are authorised to file documents online for a specific company. Think of it as a per-company password for filings — separate from your WebFiling login and separate from your HMRC Government Gateway credentials.
It is sometimes called the "auth code", "WebFiling code", or "company authorisation code" — these all refer to the same thing.
What it looks like
Six characters, mixed letters and digits — for example, K9X4P2. It is not case-sensitive. Each company has its own unique code, and the code stays the same even when directors change.
Why you need it
You need the authentication code to file anything online with Companies House:
- Annual dormant accounts (the main reason most directors need it)
- The annual confirmation statement
- Changes to directors, PSCs, registered office, or shares
- Any filing through Companies House WebFiling or third-party software like DormantFile
You do not need it for the CT600 corporation tax return — that goes to HMRC and uses Government Gateway credentials instead.
How to get one
If your company is newly incorporated, Companies House posts the code to your registered office address automatically within 5 working days. The letter is plain and easy to mistake for junk mail.
If you have never received one, or it's lost, you can request a replacement:
- Go to Companies House WebFiling
- Sign in or create an account, then add your company by company number
- Request the authentication code
- Companies House posts it to the registered office address within 5 working days
The code is sent by post only — not email — as a security measure that proves the person filing has access to the registered office.
Important things to know
- The code doesn't expire, but you can request a new one at any time (which invalidates the old one).
- It's linked to the company, not to you. If directors change, the code stays the same.
- There is no "view my code" page online. If it's lost, you must request a replacement and wait for the post.
- Anyone with the code can file on the company's behalf, including changing directors or allotting shares. Keep it secure.
- Each company has its own code. If you manage multiple dormant companies, you need a separate code for each.
When you file through DormantFile, your authentication code is transmitted directly to Companies House at submission and not stored in our database. See our security page for the full detail.
Need the full picture?
Read the complete guide: Companies House authentication code: the complete guide — covers what to do if the registered office is wrong, how ECCTA identity verification interacts with the code, formation agent edge cases, and every common "I can't find it" scenario.
Or the filing walkthrough: How to file dormant company accounts with Companies House.