HMRC Error 1046: Authentication Failure Fix

Getting "Error 1046: Authentication Failure" when submitting your CT600? This error means HMRC couldn't verify your Government Gateway credentials. Here's how to fix it.

What Causes Error 1046?

Error 1046 occurs when HMRC's system cannot authenticate your submission. The most common causes, in order:

1. Entering your HMRC username instead of your 12-digit Gateway User ID — This is the #1 cause. See Step 1 below.
2. Incorrect password — Wrong or out-of-sync password
3. UTR not linked to Government Gateway — Your company's UTR isn't authorised for online filing
4. Expired or locked account — Your Government Gateway account needs attention
5. Agent authorisation missing — If you're an accountant, your client hasn't authorised you

How to Fix Error 1046

Step 1: Check You're Using Your 12-Digit Numeric Gateway User ID

This is the most common cause of Error 1046. Your Government Gateway User ID is a 12-digit number (e.g. `123456789012`) — it is NOT your HMRC username, email address, or any other text login.

Many people confuse their HMRC username (which can be letters and numbers) with their Gateway User ID (which is always a 12-digit number). If you're entering anything that contains letters, that's the wrong credential.

Where to find your 12-digit Gateway User ID:

1. Check the email you received when you first created your Government Gateway account — the subject line is usually "Your Government Gateway user ID"
2. Or go to Government Gateway, click Problems signing in, then I've forgotten my user ID — HMRC will send it to your registered email

Step 2: Verify Your Password

1. Go to Government Gateway
2. Log in with your 12-digit User ID and password
3. If you can't log in, reset your password using Problems signing in > I've forgotten my password

Step 3: Check Your UTR is Linked

Your company's Unique Taxpayer Reference (UTR) must be linked to your Government Gateway account:

1. Log into Government Gateway
2. Go to "Manage account"
3. Look for "Corporation Tax" in your enrolled services
4. If it's not there, you need to enrol for Corporation Tax online filing

Step 4: Enrol for Corporation Tax (if needed)

If Corporation Tax isn't in your enrolled services:

1. Click "Add a tax, duty or scheme"
2. Select "Corporation Tax"
3. Enter your company's UTR (10 digits)
4. HMRC will send an activation code by post (7-10 working days)
5. Use the code to complete enrolment

Step 5: Re-enter Credentials in Clear Books Tax

Once your Government Gateway is set up correctly:

1. Go to your Clear Books Tax submission preview page
2. Click Submit Corporation Tax
3. Enter your 12-digit numeric Government Gateway User ID
4. Enter your password
5. Submit again

For Accountants

If you're filing on behalf of a client:

1. Check agent authorisation - Your client must have authorised you as their agent
2. Use agent credentials - Enter YOUR Government Gateway Agent ID and password
3. Verify client UTR - Ensure you have the correct UTR for the company

Setting Up Agent Authorisation

1. Client logs into their Government Gateway
2. Goes to "Manage account" > "Give someone else authorisation"
3. Enters your agent reference number
4. Confirms the authorisation

Still Getting Error 1046?

If you've checked everything above and still see Error 1046:

• Wait 24 hours - HMRC systems may take time to sync after changes
• Try a different browser - Clear cache or use incognito mode
• Contact HMRC - Call the Corporation Tax helpline: 0300 200 3410

Prevention Tips

• Save your credentials - Use a password manager
• Keep UTR handy - Store your company's UTR safely
• Set calendar reminders - Renew Government Gateway access annually
• Test before deadlines - Don't leave CT600 filing to the last day