Anna Soubry says 11 Tories backbenchers support her bill to keep Britain in the customs union

Theresa May is facing the biggest test of her fragile parliamentary authority as more than ten Tory MPs announced that they would vote to keep Britain in a customs union with Europe.

Two of the party’s select committee chairmen and three former ministers put their name to an amendment that would force the prime minister to put Britain’s membership of a customs union back on the negotiating table.

The announcement came hours after the cabinet’s Brexit committee signed off a policy that Jeremy Hunt, the health secretary, said excluded a customs deal with the European Union. Instead the government is expected to pursue a policy of so-called “managed divergence” where Britain agrees to follow EU rules in specific areas in return for market access but