Watch as MPs debate the controversial Internal Markets Bill that could override Brexit agreements, in the Commons
The ex-PM said he had "grave misgivings" about proposals to row back on some aspects of the Withdrawal Agreement, following Theresa May, Gordon Brown, Tony Blair and John Major in attacking the plan.
ITV News Political Correspondent Paul Brand discusses the controversial bill
It means every living former leader prime minister has criticised the Internal Markets Bill ahead of its second reading in the Commons later today.
On the Bill, Mr Cameron said: "Passing an act of Parliament and going on to break an international treaty obligation should be the very last thing you contemplate, an absolute last resort."
"So I have grave misgivings about what is being proposed."
He added however that the Bill is being introduced as a contingency, should negotiations with the EU fail to reach consensus on areas of contention such as the Northern Ireland protocol.
Prime Minister Johnson is not only facing backlash from his predecessors, but a growing number of Tory MPs have said they will rebel against the government and vote down the Bill.
Tory MP Rehman Chishti resigned as the PM's Special Envoy for Freedom of Religion or Belief over the Bill on Monday.
Announcing his resignation in a tweet, he said: "Respecting rule of law & honouring one’s word are dear to me."
Mr Johnson's former attorney general Geoffrey Cox - a Brexiteer - is the most senior MP to criticise the Bill, but he follows several other Eurosceptics in saying it's a bad idea.
Mr Cox said it would be “unconscionable” to override the Brexit divorce deal, adding how there's “no doubt” the “unpalatable” implications of the Withdrawal Agreement were known when the prime minister signed it, a time when Mr Cox was the chief law officer.
Former chancellor Sajid Javid has tweeted he would be “regretfully unable to support the UK Internal Market Bill unamended”.
He added in a statement: “One of the UK’s greatest strengths and traditions is respect for the rule of law. Our long-standing reputation for keeping our word has made us a more stable, peaceful and prosperous nation.
“Breaking international law is a step that should never be taken lightly. Having carefully studied the UK Internal Market Bill it is not clear to me why it is necessary to do so.
“While I fully backed every measure necessary to get the Withdrawal Agreement negotiated and passed by Parliament, I cannot support the UK pre-emptively reneging on that agreement. I will therefore regretfully be unable to support the Bill at its Second Reading, and urge the Government to amend it in the coming days."
He added: “The EU is certainly guilty in its negotiations of not treating the UK like the close partner we are. If it does indeed act in bad faith by reneging on the Withdrawal Agreement, then we should not only use the safeguards enshrined within that agreement, but also reserve the right to respond in kind to affirm our sovereignty and Union.
“I’m optimistic that both sides can still agree on a trade deal, but even if we do have to walk away the UK will prosper. The Prime Minister continues to have my full confidence and support as we deliver on our ambitious, post-Brexit agenda.”
On Friday Mr Johnson held a conference call with around 250 backbench MPs to convince the likes of Sir Roger Gale and Sir Bernard Jenkin to back him when the Bill is voted on in the Commons.
A backlash began to brew among MPs after Northern Ireland Secretary Brandon Lewis admitted the plan would breach international law in a “very specific and limited way”.
Mr Johnson told the MPs the Internal Markets Bill is a "necessary" way to protect the UK from EU influence after the transition period is over.
He told them clauses in the Internal Market Bill are "necessary to stop a foreign power from breaking up our country" and insisted there are still hopes of a Canada-style trade deal.
On Sunday former PM's John Major and Tony 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.
Their comments followed criticisms from Gordon Brown and Theresa May earlier in the week.
Labour MPs are also criticising the proposals, with current leader Sir Keir Starmer and former leader Ed Miliband both labelling the move "wrong".
"He is making a mistake reneging on a treaty, that will have reputational damage for the UK," Sir Keir told LBC radio.
"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."
Mr Miliband told ITV News disagreement on the Bill "is not about Remain or Leave, it's about right or wrong."
He added: "Why is it wrong? Because we are known as a country around the world for observing the rule of law.
"How are we going to strike trade deals with other countries in the future if they think they can't trust our word.
"It's the wrong thing to do, I never thought I'd see a government and come along and say we're going to disobey international law - an agreement we signed - the government should think again."
Despite the growing disquiet about the Bill, Mr Johnson's large Commons majority means he should win an expected vote of the Bill’s principles during the second reading on Monday.
But if he does win, the EU will be angry.
It has said it will walk away from free trade negotiations if the UK implements the Bill, meaning a no-deal Brexit would be the final outcome.
In a statement, the EU said: "Violating the terms of the Withdrawal Agreement would break international law, undermine trust and put at risk the ongoing future relationship negotiations."
The EU did not “accept the argument” that the UK Internal Market Bill was needed to protect the Good Friday Agreement.
“In fact, it is of the view that it does the opposite,” the statement said.
“Vice-president Maros Sefcovic called on the UK Government to withdraw these measures from the draft Bill in the shortest time possible and in any case by the end of the month.
"He stated that by putting forward this Bill, the UK has seriously damaged trust between the EU and the UK. It is now up to the UK Government to re-establish that trust.”