A new bill, promoted by Prime Minister Boris Johnson, which many have said would break international law, was angrily criticised on Monday by one of his own Brexit allies.
Geoffrey Cox, a pro-Brexit former attorney general, condemned the plan, calling it "unconscionable" and "unpalatable" that Mr Johnson could create a new UK law that overruled international law.
Opponents of the Internal Market Bill say it breaks international law but Mr Johnson has called it "absolutely vital".
Mr Cox stepped in to the argument just as former Prime Minster David Cameron also joined a long list of critics from both major parties including three other ex-PMs Tony Blair, Theresa May and John Major.
"It is unconscionable that this country, justly famous for its regard for the rule of law around the world, should act in such a way," Mr Cox wrote in The Times.
"When the queen’s minister gives his word, on her behalf, it should be axiomatic that he will keep it, even if the consequences are unpalatable.
"No British minister should solemnly undertake to observe treaty obligations with his fingers crossed behind his back.
"What ministers should not do . . . is to take or use powers permanently and unilaterally to rewrite portions of an agreement into which this country freely entered just a few months ago," he added.
Mr Cox was sacked by Johnson in February but he was also a supporter of Brexit and warned of the dangers in the plan.
"Manifest consequences included the inevitable application of EU tariffs and customs procedures to certain goods entering Northern Ireland from Great Britain and of the EU's state aid regime to the province.
"There can be no doubt that these were the known, unpalatable but inescapable, implications of the agreement. They included a duty to interpret and execute both the agreement and the protocol in good faith."
Mr Cox also added a warning to the EU: "The duty of good faith is a two-way street. The EU faces a precisely equal obligation and if, as reported, it has sought to use the Northern Ireland protocol as a lever in the trade negotiations."
Mr Cameron said he has "grave misgivings" about what is being proposed in the Internal Market Bill which the Government has admitted could break international law.
"Passing an act of Parliament and then going on to break an international treaty obligation is the very, very last thing you should contemplate. It should be the absolute final resort," he said.
The House of Commons will on Monday debate the Internal Market Bill, which the EU has demanded be scrapped. After the debate, lawmakers will decide if it should go to the next stage. A vote could be later.
The bill has plunged Brexit back into crisis less than four months before Britain is finally due to leave the EU's orbit when a post-Brexit transition period ends in December.
He has a majority of 80 in the lower house of parliament, is facing a growing revolt from some of his own lawmakers.
Ed Miliband, an opposition Labour MP, said it was hypocritcal of the government to expect people to obey the law while it was planning to break it.
Crime and policing minister Kit Malthouse said he would not resign if the government knowingly broke the law, and that he would be voting for the bill.