Boris Johnson is facing mounting criticism over his plans to introduce legislation to override his Brexit deal, as former prime ministers Sir John Major and Tony Blair criticised the threat to break international law. Major and Blair united to urge MPs to reject the legislation, saying it imperils the Irish peace process, trade negotiations and the UK’s integrity – despite the prime minister saying that Brussels could “carve up our country” without his new bill. The former Conservative and Labour leaders united to condemn Johnson’s UK internal market bill in an article for the Sunday Times. “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. “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.” The Labour leader, Sir Keir Starmer, accused Johnson of “reigniting old rows” by working to flout his own withdrawal agreement, but pledged Labour support if the prime minister addressed “substantial” concerns about the legislation. He called on Johnson to throw out clauses that could breach international law and those that have led the devolved administrations to warn of a “power grab”. “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 wrote in the Sunday Telegraph. “But if they do not, and the talk collapse, then it is their failure and incompetence that will have let the British people down.” Tory rebels suggested their numbers were growing and opinions were only hardened by Johnson’s increased rhetoric that the EU could impose a trade border in the Irish Sea. Despite Johnson’s attempts to drum up support, 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. The Commons justice committee chairman, Sir Bob Neill, who has tabled an amendment that he said would impose a “parliamentary lock” on any changes to the withdrawal agreement, said he still contended it contained “objectionable” elements. Damian Green, who was Theresa May’s deputy when she was prime minister and is backing the amendment, was also understood to not have been won over by Johnson’s argument. Sir Roger Gale also remained a vehement critic, telling Times Radio: “If anybody is responsible, if it happens, for bringing the union down, it will be [chief aide Dominic] Cummings and Mr Johnson.”