Theresa May will claim on Monday the Brexit Withdrawal Agreement is 95% completed, amid increased speculation that her leadership is under threat from Tory MPs.

The prime minister will tell Parliament the UK and EU have found “broad agreement on the structure and scope of the future relationship”.

But May will again reject the backstop proposal from Brussels that Northern Ireland remain in a customs union with the EU if a full trade agreement is not reached.

Detailing progress that has been made in the talks, May will say “taking all of this together, 95 per cent of the Withdrawal Agreement and its protocols are now settled”.

But she will add: “As I set out last week, the original backstop proposal from the EU was one we could not accept, as it would mean creating a customs border down the Irish Sea and breaking up the integrity of the UK.

“I do not believe that any UK Prime Minister could ever accept this. And I certainly will not.”

The British government has proposed instead keeping the entire UK inside the customs union until a trade deal is agreed.

But this version has been rejected by the EU, as Brussels argues it would undermine the integrity of the single market.

May also faces a fight at home, as Tory MPs are unhappy at the idea of any part of the UK remaining indefinitely in a customs union with the EU after Brexit.

The internal Tory row exploded on Sunday, as one Conservative told The Sunday Times that May was entering the “killing zone”.

Some of the PM’s critics have claimed the 48 letters needed to trigger a no confidence vote in her leadership could soon be reached.

If 158 Tory MPs then voted against her, she would be ousted as leader.

May is due to face her MPs at a meeting of the backbench 1922 committee on Wednesday.

A former Tory minister told The Sunday Times: “The moment is coming when the knife gets heated, stuck in her front and twisted. She’ll be dead soon.”

“Assassination is in the air,” one Tory MP told the paper.

One Conservative MP told the Mail On Sunday the prime minister should “bring her own noose” to the gathering of backbenchers.

The language was condemned by Tory MPs on both the pro-Brexit and pro-Remain sides of the party.

Former Cabinet minister Theresa Villiers, a Brexiteer, said the comments about the PM in the newspapers were “disturbing”.

Backbencher Johnny Mercer last week described May’s government as a “shitshow”.

But he hit out at how some of his colleagues had chosen to make their criticisms.

“I would never use the language of some MP’s against the PM today, and condemn it utterly,” he tweeted.