Tony Blair and Sir John Major have teamed up to accuse Boris Johnson of "shaming" the UK by proposing legislation which undermines his own Brexit deal.
The two former prime ministers, who unsuccessfully opposed Brexit in the 2016 EU referendum, claim the PM's UK Internal Market Bill will damage the Irish peace process and trade talks.
Their attack comes ahead of MPs debating the bill in the Commons on Monday, with Mr Johnson facing a sizeable Tory backbench rebellion and an attempt by rebel MPs to amend the bill.
In a highly significant intervention, the Labour leader, Sir Keir Starmer, has revealed that if the government "fixes" MPs' concerns - allows amendments to the bill - then Labour is prepared to back it.
Writing in The Sunday Times, the two ex-prime ministers said: "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.
"As the world looks on aghast at the UK - the word of which was once accepted as inviolable - this government's action is shaming itself and embarrassing our nation."
Mr Blair and Sir John, who admitted that when they opposed Brexit in 2016 they were dismissed as "out of touch and out of time has-beens", bitterly attacked Mr Johnson personally.
"We have become so inured to the unending Brexit saga of misinformation and misdirection that the hollow claims, empty promises and emptier threats have lost their power to shock," they wrote.
"Yet what is being proposed now is shocking. How can it be compatible with the codes of conduct that bind ministers, law officers and civil servants deliberately to break treaty obligations?
"As we negotiate new trade treaties, how do we salvage credibility as "global Britain" if we so blatantly disregard our commitments the moment we sign them?
"The government seeks to do so by the extraordinary pretence that breaking international law is necessary to 'save the Good Friday agreement', which has given us peace in Northern Ireland for more than two decades and utterly changed the relationship between the UK and its nearest neighbour, the Republic of Ireland.
"We disagree. The government's action does not protect the Good Friday agreement - it imperils it."
Responding to their comments, former justice minister David Gauke told Sky News that he thinks the bill is "clearly a breach of international law".
He said: "It will make it harder for other countries, not just the EU, but around the world, to trust the UK in future, and that is not good news for the country.
"And we will be diminished as a consequence, and actually British public will pay a price for that, because if we can't get deals, if our word can't be trusted, then it will be harder for us to engage with the rest of the world.
"It also poisons the negotiations with the EU, it makes it harder for us to reach a deal before the end of this year, which will be very bad news for the country."
He added: "The government had admitted that it's a breach of international law. International law is law, and is therefore undermining the rule of law.
"Either he (Boris Johnson) failed to understand what he had signed, or he signed something in bad faith with the intention of ripping up his agreement it's one or the other. Frankly either is plausible, but it either suggests mendacity or incompetence."
Mr Gauke said he believed it would get through the House of Commons, but not the House of Lords.
Tony Blair and Sir John Major's attack is unlikely to trouble Mr Johnson and his inner circle of Brexiteers, however, who will argue that the former prime minister's views were rejected by voters in the referendum.
Of more concern to the prime minister is the opposition from Conservative MPs and support for an amendment to the bill which attempts to give parliament a veto on moves to change the EU Withdrawal Agreement.
Writing in The Sunday Telegraph, Sir Keir accused the prime minister of turning back the clock and reigniting old rows and urged him: "We should be getting on with defeating this virus, not banging on about Europe.
"Get on with Brexit and defeat the virus. That should be the government's mantra. Labour is prepared to play its part in making that happen.
"If the government fixes the substantial cross-party concerns that have been raised about the Internal Market Bill, then we are prepared to back it.
"But if they do not, and the talks collapse, then it is their failure and incompetence that will have let the British people down."
Sir Keir also wrote: "I am not blind to the fact this could be part of the government's negotiating strategy in the final weeks of talks.
"However, the decision to propose a bill last week that would break an international agreement we signed a matter of months ago is wrong.
"Many former leaders of the Conservative Party have spoken out to say so. And I suspect the prime minister agrees with them and knows his actions are counterproductive.
"First, public trust has been broken. The deal the prime minister promised would be done is now being reopened. The divisions within the Conservative Party are being reignited.
"And at a time when we are all doing everything we can do to follow social distancing rules, ministers are on the television and radio justifying why it is ok for them to break international law. It is as ludicrous as it is frustrating.
"We have broken the trust of our global partners too. I believe we can be a proud nation on the world stage and a moral force for good.
"We can work with our international partners to strike the best possible trade deals. But, the prime minister risks holding Global Britain back. How can we get the best trade deals when we cannot be trusted to keep our word?".