Theresa May has insisted Brexit means quitting the EU customs unionso the UK can strike free trade deals with other countries.
But this means that customs checks on goods will probably need to be carried out at the bordercreating the spectre of long border queues.
Critics of the PM's approach say the UK should stay in a customs union with the bloc to avoid these hard border controls.
Below are three customs deals the EU has done with countries outside the bloc:
The Norway Option:
Norway voted narrowly against joining the EU in 1994, but shares a 1000-mile border with Sweden which is in the bloc.
The Norwegian government decided to negotiate a deal which gave it very close ties with the EU.
It is part of the EU single market which means it must accept EU rules on the free movement of people.
But it is not in the customs unionmeaning it sets its own tariffs on customs coming from outside the EU and so must carry out border checks.
There are some 1,300 customs officials who are involved in policing the border with Sweden, and have invested substantial amounts in technology to make these as quick and smooth as possible.
They have IT systems which pre-declare goods to customs and they are developing a system which will allow lorries carrying pre-declared goods to be waved through.
Norway also pays large amounts into the EU budget and is governed by the court of the European Free Trade Association (EFTA).
Switzerland (border pictured) is not in the EU customs unionwhich means that checks on goods crossing over the border from non-EU countries are carried
The Switzerland Option:
Switzerland is one of the EU's longest-standing trading partners, but the country voted against joining the bloc in 2001.
It is a member of the EU single market and has signed up to the Schengen areameaning it must accept free movement rules and does not carry out passport check on other member countries.
But it is not in the EU customs unionwhich means that checks on goods crossing over the border from non-EU countries are carried out.
The situation tosses up some anomalies. For instance, a passenger travelling through Geneva Airport can rent a car on the French side of the border for around half of the cost of renting it on the Swiss side.
Border checks are carried out on goods but customs officials say they use intelligence to carry out spot checks, which can be carried out several miles from the border.
However, there can be long delays as goods are checked at the border.
The Turkey Option:
Turkey(its border with Bulgaria pictured) has long eyed up membership of the EU and first tried to start the lengthy application process to join in 1987.
Turkey has long eyed up membership of the EU and first tried to start the lengthy application process to join in 1987.
The country signed a customs union with the bloc in 1995a move Turkey's rulers hoped would be a stepping stone on the way to full membership.
Turkey's hopes to join the bloc faded over the past few years and have been all but abandoned under President Erdogan after he instigated a major purge of political opponents in the wake of the failed coup against him in 2016.
Under its customs union Turkey must follow EU rules on the production of goods without a say in making them.
It also means that Turkey can only strike free trade deals on goods which are negotiated by Brussels.