Tory former prime minister David Cameron has joined forces with other ex-premiers in expressing "misgivings" about overriding the Brexit divorce deal.
Though Mr Cameron did not go quite as far in his criticism as Sir John Major, Tony Blair, Gordon Brown and Theresa May, the move piles further pressure on Prime Minister Boris Johnson amid a growing Tory revolt over controversial new proposed legislation grew.
Mr Johnson is attempting to push through the Internal Market Bill, which the Government has said would breach international law, as ministers insist it is a "safety net" if no trade deal is agreed with Brussels before the conclusion of the Brexit transition period at the end of the year.
In a statement Mr Cameron said: "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 an absolute final resort.
"So, I do have misgivings about what's being proposed.
"But, I would just make this point.
"So far what's happened is the Government has proposed a law that it might pass, or might not pass, or might use, or might not use depending on whether ... certain circumstances do, or do not appear."
Mr Cameron added: "And, of course, the bigger picture here is that we are in a vital negotiation with the European Union to get a deal and I think we have to keep that context, that big prize in mind."
The Government has insisted the Internal Market Bill will ensure barrier-free trade across the UK after the Brexit transition period concludes at the end of the year.
However, Mr Johnson's former attorney general Geoffrey Cox also spoke out against the plans.
He told Times Radio that the Government "knew" what it was signing up to when it ratified the Withdrawal Agreement.
"What I can say from my perspective is we simply cannot approve or endorse a situation in which we go back on our word, given solemnly not only by the British Government and on behalf of the British Crown, but also by Parliament when we ratified this in February, unless there are extreme circumstances which arrive involving a breach of duty of the good faith by the EU.
"In those circumstances, there are then lawful remedies open to us and it is those we should take rather than violating international law and a solemn treaty.
"The breaking of the law leads ultimately to very long-term and permanent damage to this country's reputation and it is also a question of honour to me – we signed up, we knew what we were signing.
Mr Cox said there are "legal measures available" for tackling any attempt by the European Union to prevent food being transported between Britain and Northern Ireland.
Mr Cox said the Government had not fully explained the circumstances in which powers in the Internal Market Bill would be used.
He said: "I think the fundamental problem at the moment is that it is not clear the circumstances in which the powers taken by the Bill would be used.
"The Government thus far has not given any definition to those circumstances.
"If the powers are to be used simply to nullify the foreseeable and ordinary consequences of an agreement we signed, that to me is simply to go back on an agreement that both the British Government signed solemnly and Parliament itself ratified in February.
"I think it is wrong that the British Government or our Parliament should renege on an agreement on which we gave our solemn word."
The Brexiteer warned he would not back the UK Internal Market Bill unless ministers dispel the impression they plan to "permanently and unilaterally" rewrite an international agreement.
Mr Cox's intervention came ahead of MPs debating the legislation on Monday, when the Bill returns to the Commons amid growing criticism that breaching international law would jeopardise the UK's standing in the world.
Justice Secretary Robert Buckland had earlier said the controversial powers amounted to a "break the glass in emergency provision if we need it" and said he did not believe they would be used.
Mr Johnson warned that Brussels could "carve up our country" without his new Bill, as he stepped up his rhetoric as senior Tories prepared to rebel against the legislation.
Outrage at the Bill has come from across the political spectrum, Sir John and Mr Blair united to urge MPs to reject the "shaming" legislation, saying it imperils the Irish peace process, trade negotiations and the UK's integrity.
"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," they wrote in the Sunday Times.
Protocol on IE/NI is not a threat to the integrity of the UK. We agreed this delicate compromise with @BorisJohnson & his gov in order to protect peace & stability on island of Ireland. We could not have been clearer about the consequences of #Brexit [1/2]
— Michel Barnier (@MichelBarnier) September 13, 2020
Despite Mr Johnson's attempts to drum up support, Tory rebels suggested their numbers were growing and opinions were only hardened by Mr Johnson's increased rhetoric.
The Prime Minister, with a large Commons majority, should win an expected vote of the Bill's principles during the second reading of the Bill on Monday.
But a rebellion could come later with Commons justice committee chairman Sir Bob Neill's amendment, which he said would impose a "parliamentary lock" on any changes to the Withdrawal Agreement.