What is the customs union and why does it matter?

Francois Lenoir | Reuters

Britain’s Prime Minister Theresa May arrives at a European Union leaders summit in Brussels, Belgium, March 22, 2018.

“We are leaving the customs union, we will have an independent trade policy and we will strike trade deals around the world,” May’s spokesman said this week.

That sounds pretty clear-cut, but the U.K. parliament will have its say and could make May’s authority difficult to maintain.

Britain’s upper house, the House of Lords, decisively voted to retain a customs union this week and the House of Commons will debate and conduct a non-binding vote on Thursday.

May and her supporters could face an embarrassing defeat in parliament if members of her own Conservative Party ally with others to press for a continuation of the customs union.

Right-wing Brexiteers want May to make the debate a “back me or sack me” issue in order to persuade her own MPs from voting against her. The vote is non-binding and May has made no pledge to resign over it, but the pressure will be ratcheted up should she lose.

If that happens but May stays on as leader, the war of words currently ongoing within the Conservative Party could explode into resignations and leadership challenges.