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.

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,” he said.

“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.”

I think it is wrong that the British Government or our Parliament should renege on an agreement on which we gave our solemn word

He added: “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 that they plan to “permanently and unilaterally” rewrite an international agreement.

Simon Hoare, Tory chairman of the Commons Northern Ireland Affairs Committee, said Mr Cox’s condemnation of the Government’s plans could not be ignored.

He said: “Geoffrey is a man for whom respect for the rule of law (and its defence) is paramount. His intervention cannot be overlooked, ignored or swatted away.”

He highlighted the influential role the former attorney general played as a “lightening (sic) rod” among backbench MPs during Mrs May’s Brexit battles.

“I remember hearing lots of colleagues saying at the time: ‘if Geoffrey says it’s no dice; I’m not playing’. Lots saying ‘if Cox unhappy I’m not going for it’.

“As a committed Brexiter he should remain an important lightening rod.”

Meanwhile, Tory MP Rehman Chishti has resigned as the Prime Minister’s special envoy on Freedom of Religion or Belief (FoRB) in opposition to the Internal Market Bill.

He tweeted: “I’ve written to the PM resigning as PM’s Special Envoy on FoRB. I can’t support Internal Market Bill in its current form, which unilaterally break UK’s legal commitments.

“As an MP for 10yrs & former Barrister, values of respecting rule of law & honouring one’s word are dear to me.”

Labour leader Sir Keir Starmer said the Government’s position on the Internal Market Bill is damaging to the country and would cause “reputational damage”.

He told LBC: “Here we are on the world stage for the first time in many years on our own and what’s the first thing we do? We break a treaty.

“It’s basic stuff – if you say to other nations we agree something and a few months later you say no we don’t, the chances are they aren’t going to trust you going forward.”

The interventions 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 and 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.

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.