My trademark is registered in the EU trademark register and does not need to be renewed yet

Then you don’t have to do anything. From 2021, you’ll have automatically and gratuitously both an EU trademark and a national trademark in the United Kingdom.

My EU trademark registration has been applied, but the  trademark has not been registered yet

In that case, you will have 9 months starting from January 1, to confirm your ongoing application for the United Kingdom. Make sure you make your choice on time. Do you confirm the  application for the United Kingdom? Then you will pay an additional fee for this, on top of the fee for the application of your EU trademark.

My trademark registration is about to expire and needs to be renewed

In addition to renewing your EU trademark, you will then also need to choose if you want to keep your trademark rights in the United Kingdom. 

Was the expiration date of the rights before January 1, 2021?

Then you had the choice to renew the EU trademark registration before Brexit. Your EU trademark has then meanwhile automatically and gratuitously been converted to a national  trademark in the United Kingdom.

Will your rights expire after January 1, 2021?

In that case, you will need to confirm your EU trademark for the United Kingdom separately so that a national trademark comes into existence for the United Kingdom. For the renewal of your  trademark in the United Kingdom, a separate fee is applicable.