Boris Johnson, the foreign secretary, and Liam Fox, the trade secretary, are likely to lead a fight to leave the customs union

Brexiteers warned Theresa May that she could face a leadership challenge if she conceded that Britain should stay in a customs union with Europe.

Downing Street looks likely to duck a fight next week when the first of a series of votes is held. Julian Smith, the Tory chief whip, has declared Thursday’s vote on a non-binding motion a one-line whip, meaning that attendance is not compulsory for the party’s MPs.

Mrs May will have to overcome Brexit challenges on customs before the summer recess. There are warnings that MPs may start sending no-confidence letters to Sir Graham Brady, chairman of the backbench Conservative 1922 Committee. He must start a no confidence vote if he receives 48 letters.

A key cabinet committee will decide a