Barnier has suggested that it is possible that the EU may not actually grant a long article 50 extension unless May backs a customs union. He has already stated that the UK will remain an ally and a partner, also that an orderly withdrawal agreement has been the EU’s goal.
If the UK wants to leave the EU in an orderly fashion, the only way to do so is on the basis of the withdrawal agreement. He says the UK and the EU could add to the free trade agreement already proposed in the PD a customs union. This could be added rapidly, “within a few hours or days”, he says.