Русско-англо-русский юридический словарь

"Accession" is the act whereby a state accepts the offer or the opportunity to become a party to a treaty already negotiated and signed by other states. It has the same legal effect as ratification. Accession usually occurs after the treaty has entered into force. (Glossary of terms relating to Treaty actions - UNTC)

The UK’s accession to the treaty will complement the existing UK-Canada Trade Continuity Agreement (TCA) which commits both nations to high standards when it comes to tackling issues including climate change and supporting free and fair trade. (BBC)