These links are to assist tax agents. 

Login to HMRC's old main agent account

This is where you can sign in for things like your personal or business tax account, authorise clients, access services such as Self Assessment, Corporation Tax, PAYE for employers and old system VAT.

https://www.gov.uk/log-in-register-hmrc-online-services

Login New Agent Services Account

Here you can move clients into Making Tax Digital for VAT and Making Tax Digital for Self Assessment (there is a pilot running until 2026) and authorise clients for those services.

https://www.gov.uk/guidance/sign-in-to-your-agent-services-account