Portsmouth MP Penny Mordaunt locked in high-stakes Brexit meeting with Theresa May

The Portsmouth North MP was the first to arrive at the prime minister's country retreat, Chequers, to begin a gruelling 12-hour session to thrash out the wording of a key part of the UK's blue print to leave the EU.

Locked in the country home with their phones taken away, all of the PM's cabinet ministers were tasked with approving a plan which could see the UK sign up to EU rules on agriculture and food, potentially making it much harder to strike a post-Brexit trade deal with the US.

The proposal, set out by Mrs May in a document more than 100 pages long and distributed to ministers only on Thursday, has sparked fury in the Leave camp and prompted speculation that cabinet Brexiteers, like Ms Mordaunt, could be ready to walk out of the government.

Mrs May wants her team to unite behind a comprehensive offer to the EU to be set out in a White Paper for publication next week. She said ministers had '˜an opportunity - and a duty' to reach agreement on a plan which would command the support of both the public and Parliament.

Downing Street insisted it is '˜categorically untrue' that the post-Brexit relationship with the EU envisaged by Mrs May would make a trade deal with the US impossible.