The application process is free and simplified, and U.K. citizens in Hungary have until the end of 2021 to apply. In the meantime, existing documents remain valid. Everyone needs to apply, even if they already have a registration certificate or permanent residence card. For more information, please watch the message of Paul Fox, British Ambassador to Hungary. 

Full details on applying are available on the website of the immigration authority.

Information on the actions British citizens have to take is available in the Living in Hungary Guide of the Foreign Office.