Three cabinet ministers warned Theresa May during private talks on Brexit at her Chequers retreat last week that her government could collapse this year.

Julian Smith, the chief whip, told May there was a “very real threat” that Labour could unite with 15 to 20 Tory rebels to defeat the government on their decision to rule out membership of a customs union.

Senior ministers say there are discussions about whether the prime minister should turn the vote into a confidence issue, threatening a general election if Tory MPs vote with the opposition.

In the same discussion Karen Bradley, the Northern Ireland secretary, warned May that the Democratic Unionists propping up the government should not be relied upon to “turn up” and vote to save her.