As British Prime Minister Theresa May prepares for a key week for her plans over Brexit and Britain's membership of the customs union, what is the debate about?

What is the EU customs union?
The EU's customs union is a trading bloc comprising the 28 member countries within which goods flow freely across borders without tariffs.

Common external tariffs are applied to goods imported from countries from outside the bloc. Any trade deals with countries outside the bloc are agreed centrally with Brussels.

Why is it being talked about?
Whether to stay part of the EU's customs union or to leave it and strike a trade deal with the EU from outside, such as Canada or Norway, is one of the choices Britain must make as part of Brexit.

MPs are set to debate the Government's Brexit Trade Bill this week ahead of a key vote in May.

Why should the UK stay in it?
Supporters of staying in the customs union argue that it is the safest option for the economy as it will allow goods and trade to continue to flow without barriers when Britain leaves the EU.

Why should it leave?
Advocates of leaving the customs union argue that doing so will free Britain to strike its own deals with countries around the world, potentially increasing jobs and boosting the economy.

Who is in favour of remaining?
Chancellor Philip Hammond and Business Secretary Greg Clark are said to favour remaining in the customs union.

Labour's Chuka Umunna and Conservative MP Anna Soubry are prominent supporters, while the CBI business group has also pushed for the UK to retain membership.

Who is in favour of leaving?
Foreign Secretary Boris Johnson, Brexit Secretary David Davis and Trade Secretary Liam Fox are said to be pushing to leave the customs union and arrange a union with the EU from outside.

What does the UK government say?
Theresa May has made leaving the customs union a central commitment in her Brexit strategy and Downing Street sources have insisted there will be no backsliding on the issue.