Forty-three Brexiteer Tory MPs are set to vote against any amended EU-approved exit treaty put to the House of Commons, and are urging Boris Johnson to take the UK out of the EU on October 31st without a deal.

Mr Johnson is the frontrunner in the Tory leadership race, with the next party leader and prime minister expected to be announced next week. The former foreign secretary and Leave campaigner said that while he would take the UK out of the EU without a deal if necessary, he would first attempt to renegotiate the exit treaty with Brussels to remove the controversial Irish backstop and pass it in the Commons.

According to The Sun, 43 Tory MPs, members of the grassroots Stand up 4 Brexit group, are planning to tell Mr Johnson that they will block any tweaked Withdrawal Agreement Bill in the House of Commons and demand a clean, no-deal exit at the end of October.

The group includes former Conservative Party leader Iain Duncan Smith, former Secretary of State for Work and Pensions Esther McVey, former Brexit secretary David Davis, former Brexit minister Suella Braverman, and former Cabinet minister John Whittingdale.

Mr Whittingdale wrote for the tabloid: “The Withdrawal Agreement, negotiated by Theresa May, is dead.

“Having been defeated three times in Parliament, including the biggest defeat any government in our history has ever suffered, it now must be abandoned and we must start again.”

If the numbers reported by The Sun are correct, the 40-plus MPs could force a defeat of a Johnson government’s exit bill.

The call comes as a leaked report emerged of a tense meeting between Brexit secretary Stephen Barclay and the EU’ chief Brexit negotiator Michel Barnier last week, with Mr Barclay reportedly telling Mr Barnier five times that the withdrawal treaty is “dead”.

“He told Barnier that the withdrawal agreement was dead — not once but five times,” a senior EU diplomat told The Times. “If this is what is coming then we will be heading for no deal very quickly.”

Brussels bureaucrats and European leaders have maintained that the withdrawal treaty, agreed by the outgoing prime minister, Theresa May, cannot be negotiated, particularly with regards to the Irish backstop. The German defence minister tipped to be the next president of the European Commission, Ursula von der Leyen, said that while she is open to granting the UK another Brexit delay, she said the treaty offered to May is “the best and only possible deal for an orderly withdrawal”.

The backstop is the major sticking point for Brexiteers and the Democratic Unionist Party, the conservative, Northern Irish party propping up the minority Tory government. Forced upon the UK to prevent a so-called ‘hard border’ on the island of Ireland, it is intended to come into effect should London and Brussels fail to agree a future trading arrangement by the end of the transition period in December 2020.

It is feared that should the backstop be activated, it would lock the whole United Kingdom into permanent regulatory alignment with the EU, or if not, would result in Northern Ireland being locked into the Customs Union with EU member-state the Republic of Ireland whilst Great Britain diverged, creating a customs border in the Irish Sea and ultimately threatening the integrity of the Union.

MPs and Leave campaigners have voiced other concerns about the EU-approved withdrawal treaty, notably the long transition, or ‘implimentation’ period lasting until the end of 2020, during which the UK cannot initiate its own bilateral trade deals with third nations, such as the United States. Other concerns are that the ‘temporary’ arrangements become a blueprint for a future UK-EU accord, resulting in a soft Brexit where the UK is closely aligned to the EU in terms of regulations, which would permanently damage the possibility of free and fair trade with the rest of the world.