Brexit / Key Terms / What is the Customs Union? The Simple Politics guide to the Customs Union - and some of the arguments for and against. An agreement between the EU countries (plus Monaco and some British territories) to remove trade barriers so that when products travel between EU countries, there are no customs checks. However, when products come into the EU from countries outside, they will be checked and taxed - at a rate decided on by the EU, not individual countries. As part of Brexit, Theresa May wants to leave the customs union and form a separate customs agreement which would include no customs checks at the UK border for EU goods.