BORIS Johnson’s plans to override the Brexit deal he agreed with Brussels threaten the Irish peace process, John Major and Tony Blair have warned.

In a high-profile intervention, the two former political rivals came together to urge MPs to reject the Internal Market Bill, which heads to the Commons today for its second reading.

There’s three days of debate scheduled, with votes planned tomorrow and on Wednesday.

They said the legislation imperils the Irish peace process, trade negotiations and the UK’s integrity.

In a joint article in the Sunday Times, the two former prime ministers said: “We both opposed Brexit. We both accept it is now happening.

“But this way of negotiating, with reason cast aside in pursuit of ideology and cavalier bombast posing as serious diplomacy, is irresponsible, wrong in principle and dangerous in practice,” they said.

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“It raises questions that go far beyond the impact on Ireland, the peace process and negotiations for a trade deal – crucial though they are. It questions the very integrity of our nation.”

Their intervention means that four out of the five living former prime ministers have voiced concerns about the bill.

Both Gordon Brown and Theresa May have been critical.

Only David Cameron has kept silent so far. Though with the paperback edition of his memoirs released on Thursday, it’s thought he might speak out then.

DUP East Antrim MP Sammy Wilson said Blair and Major’s criticism was “bunkum”.

He said the Internal Market Bill was “far from perfect”, but added: “The suggestion by the hero of the peace process brigade that the bill rips apart the Belfast Agreement is complete and utter bunkum without any factual basis.

“They need to explain how making it easier for Northern Ireland to do business with our biggest market undermines the Belfast Agreement.”

Meanwhile, Labour leader Keir Starmer said his MPs would only support the bill if Johnson addressed “substantial” concerns.

He said that would mean scrapping clauses that could breach international law and the changes that have led to accusations of a “power grab” from the governments in Scotland and Wales.

“Labour is prepared to play its part in making that happen. If the Government fixes the substantial cross-party concerns that have been raised about the Internal Market Bill, then we are prepared to back it,” Starmer said.

“But if they do not, and the talks collapse, then it is their failure and incompetence that will have let the British people down.”

There is a sizeable Tory rebellion growing against the bill.

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The Tory chairman of the Commons defence committee, Tobias Ellwood, said on Saturday he could not support the legislation without it being changed.

“Already this bill is damaging brand UK, diminishing our role-model status as defender of global standards.

“As we go to the wire, let’s see more British statecraft – less Nixonian Madman Theory,” he tweeted.

Roger Gale, who also remained a vehement critic, told Times Radio: “If anybody is responsible, if it happens, for bringing the Union down, it will be [chief aide Dominic] Cummings and Mr Johnson.”

At least 20 Tory MPs are expected to back an amendment tabled by Bob Neill, the chair of the justice select committee.

Meanwhile, the French have made clear they are preparing for a No-Deal Brexit. Clement Beaune, the French Europe minister, said the EU remained “very united” and “will remain so. If the British multiply the knife blows in the contract, with the idea of ​​dividing Europe, that will not work”.