Senior Conservatives are not backing down on their rebellion against Boris Johnson’s controversial Brexit legislation despite his warning Brussels could “carve up our country” without it.

The Prime Minister’s bid to win support for the Bill that overrides parts of his own divorce deal was joined by Cabinet Office Minister Michael Gove warning it is needed to protect the “integrity” of the UK.

They argued the legislation that could breach international law and has prompted the EU to threaten legal action during trade negotiations is needed to prevent a trade border in the Irish Sea.

However, Tory rebels suggested that opinions were only hardened by Mr Johnson’s conference call with backbenchers and believed support was growing for their amendment to the UK Internal Market Bill.

Increasing pressure on MPs, former prime ministers Sir John Major and Tony Blair united to urge them to reject the legislation, saying it imperils the Irish peace process, trade negotiations and the UK’s integrity.

The Conservative and Labour grandees wrote in 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.

“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.”

Despite Mr Johnson’s efforts, Tory chairman of the Commons defence committee Tobias Ellwood said on Saturday that 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.

Commons justice committee chairman Sir Bob Neill, who has tabled an amendment which he said would impose a “parliamentary lock” on any changes to the Withdrawal Agreement, said he still contends it contains “objectionable” elements.

He told the PA news agency that the Prime Minister’s speech had “not at all” changed his mind, and said the “hard language” from both sides of the negotiations only strengthened the case for his opposing legislation.

“He set out his case but it hasn’t changed anything that I think. I’m confident that our amendment is still getting support,” Sir Bob said.

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 Mr 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.”

The Prime Minister spoke with around 250 MPs on Friday to try to drum up support for the Bill, and warned them against a return to the “miserable, squabbling days of last autumn” over Brexit.

And, in an incendiary article for The Telegraph, Mr Johnson said Brussels was threatening to use an “extreme interpretation” of the Northern Ireland Protocol to impose “a full-scale trade border down the Irish Sea” that could stop the transport of food from Britain to Northern Ireland.

Both Ireland and the EU have warned that Mr Johnson’s plans pose a serious risk to the peace process rather than protecting the Good Friday Agreement.

But he doubled down and argued it is “crucial for peace and for the Union itself” and said voting the Bill down would reduce the chances of a trade deal with the EU, which is hanging in the balance.

He asked MPs to help him “remove this danger to the very fabric” of the UK by taking away “the theoretical power to carve up our country”.

Mr Gove joined the Prime Minister in urging support for the Bill ahead of a Commons debate on Monday.

“We’re doing our part – generously – to help protect the EU’s own single market, but we’re clear that what we can’t have even as we’re doing all that is the EU disrupting and putting at threat the integrity of the United Kingdom,” Mr Gove told BBC Breakfast.

Mr Gove conceded that “we are reaching a crunch moment”, but insisted “we have got the support of our own MPs”.