The UK’s place in a customs union seems to be the biggest test facing those in charge of the Brexit negotiations – while the most hardline Brexiters seem to want the UK to cut all ties with the EU, Michel Barnier, the EU’s chief negotiator, has conceded that border checks within the UK may be necessary in the future as Brussels published a draft withdrawal agreement under which the Northern Ireland would effectively stay in the single market and customs union after Brexit.
But what is a customs union, and why does it matter?