It follows Brexit Secretary David Davis’s resignation over Theresa May’s proposals for trade with the EU.

The prime minister is currently addressing backbench Conservatives MPs, many of whom share the two former ministers’ concerns.

Mrs May claims her plans are the “the best way to honour” the result of the 2016 EU referendum.

The BBC’s political editor Laura Kuenssberg said Mr Johnson’s exit had turned an “embarrassing and difficult situation for the PM into potentially a full-blown crisis”, fuelling speculation about a leadership challenge.

Mrs May’s official spokesman said she would fight any attempt to oust her if the required 48 Tory MPs called for a contest.

The UK is due to leave the European Union on 29 March 2019, but the two sides have yet to agree how trade will work between the UK and the EU afterwards.

There have been differences within the Conservatives over how far the UK should prioritise the economy by compromising on issues such as leaving the remit of the European Court of Justice and ending free movement of people.

Theresa May only has a majority in Parliament with the support of the 10 MPs from Northern Ireland’s Democratic Unionist Party, so any split raises questions about whether her plan could survive a Commons vote.

She took her entire cabinet to her country residence on Friday to try and get agreement on a UK vision for post-Brexit relations.

An agreement was announced after the 12 hour meeting, but many Brexiteers have been unhappy with the deal which they think will lead to the “worst of both worlds”.