Prime Minister Theresa May's plan for Britain to leave the EU customs union when it quits the bloc — and not join a similar arrangement — has become a flash point in the Brexit process, setting up a possible parliamentary defeat that could force her government into a damaging policy U-turn.
The European Union's customs union is a legal arrangement in which all member states participate. Some other countries have also joined under varying conditions, including Turkey, but May has ruled this out for Britain.
Members of a customs union apply the same tariff to imports from outside the union, and apply no tariffs to goods from other members of the union.