Downing Street is facing a showdown as the House of Commons prepares to debate the Internal Market Bill, which the EU has demanded Boris Johnson scrap by the end of September.
Last week, the government admitted that its plan to reinterpret the special Brexit arrangements for Northern Ireland will break international law.
In the new internal market bill, the government is expected to unveil plans for domestic powers to govern part of the Northern Ireland protocol, something that has threatened to disrupt Brexit talks that resumed in London on Tuesday.
A number of Tory MPs intend to abstain in Monday’s vote, with up to 30 expected to back the amendment tabled by Sir Bob Neill, the chair of the justice select committee.
The EU is ready to impose a no-deal Brexit, while Britain has dismissed an ultimatum from Brussels to scrap the main parts of the bill by the end of September.
The EU says it cannot trust those who break agreements and that if the bill is not effectively scrapped there will be no trade deal to cover everything from car parts to food.
Former Attorney General Geoffrey Cox has said he will vote against the government's attempts to override the Brexit withdrawal agreement when it comes before the Commons.
He has accused Boris Johnson of doing "unconscionable" damage to Britain's international reputation.
“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,” Johnson’s former Cox said in The Times.
“No British minister should solemnly undertake to observe treaty obligations with his fingers crossed behind his back,” added Cox.
The shadow Cabinet Office minister, Rachel Reeves, confirmed that Labour would also vote against the bill in its current form.
Former prime ministers Sir John Major and Tony Blair also criticised the prime minister’s threat to break international law over the weekend. In a joint article written for the Sunday Times, the pair urged MPs to reject the legislation, saying it imperiled the Irish peace process, trade negotiations and the UK’s integrity.
Meanwhile, a new war of words broke out on Twitter between Downing Street’s chief negotiator, David Frost, and his EU counterpart, Michel Barnier, over the Northern Ireland protocol, under which it would continue to enforce EU customs and follow product standards rules to prevent a hard border on the island of Ireland.
The UK left the EU on 31 January, having negotiated and signed the withdrawal agreement with the bloc.
The two sides are now in the closing weeks of negotiations for a post-Brexit trade before the transition period ends on 31 December.
A key part of the withdrawal agreement - which is now an international treaty - was the Northern Ireland Protocol, designed to prevent a hard border returning to the island of Ireland.
Frost claimed the EU had made it clear there is no guarantee it will add Britain to its list of approved third countries for food imports. But Barnier said it needed details from the UK on its future health standards for food, plant and animal origin products for export, known as sanitary and phytosanitary standards.
The Internal Market Bill proposed by the government would override that part of that agreement when it came to goods and would allow the UK to modify or re-interpret "state aid" rules on subsidies for firms in Northern Ireland, in the event of the two sides not agreeing a future trade deal.
There were new calls from Brussels and EU capitals on Sunday for the internal market bill to be dropped.
Clément Beaune, France’s EU affairs minister, said it would be inconceivable for London to enact a bill that would partly contradict the agreement ratifying its divorce from the EU.
The EU has warned the UK it could face legal action if it does not consider dropping the controversial elements of the Internal Market Bill by the end of the month.