If you decide to start a partnership, you must inform HMRC of your decision, regardless of whether or not you already complete a self-assessment tax return.
It is best to register with HMRC as soon as you start trading. The latest you can register is by 5th October in your business' second tax year
HMRC can issue penalties for late registration so it’s in your best interests to do this on time!
Registering with HMRC is very straight forward, you simply need to complete your partnership business registration form, which can be found by clicking here.