LONDON (BLOOMBERG, REUTERS) - Prime Minister Boris Johnson will put his plan to break international law over Brexit to its first debate and vote in Parliament on Monday, amid a growing rebellion from his MPs and further criticism from former premiers and European Union leaders.

Johnson's legislation, which would allow the UK to unilaterally override parts of the divorce treaty it signed with the EU, will be given its first airing in the House of Commons on Monday.

The EU has threatened legal action unless he backs down by the end of the month, a request his government has rejected.

Johnson is likely to get the Internal Market Bill over its first voting hurdle on Monday evening and at this stage his plan seems likely to become law, although bigger rebellions in the House of Commons are expected next week when MPs vote on proposals to re-write the most controversial parts of the legislation.

The risk for Britain is that the controversy around Johnson's plan increases the prospect of ending the Brexit transition period without a free-trade accord with the EU, causing an economic shock as tariffs and quotas are introduced on commerce with its biggest trading partner.

Fixing his Brexit plan into law would be the latest escalation of the dispute with the EU, which has accused the UK government of undermining trust and reneging on an international treaty.

Johnson's immediate challenge is to marshal his Conservative MPs into supporting him and see off a revolt from his own lawmakers who are seeking to amend the Bill.

In a sign that the rebellion is growing, Johnson's former attorney-general Geoffrey Cox joined the ranks opposed to his plan, accusing him in the Times newspaper 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," Cox said in The Times. "No British minister should solemnly undertake to observe treaty obligations with his fingers crossed behind his back,"said Cox, who was sacked by Johnson in February.

More than a dozen Conservative MPs had been waiting for Cox's opinion before deciding whether to back the bill, the newspaper reported.

Cox's stark intervention followed further disapproval of Johnson's actions over the weekend.

Former prime ministers John Major and Tony Blair wrote a joint opinion piece in the Sunday Times saying his plan is "shocking" and imperils the Good Friday accord that led to more than two decades of peace in Northern Ireland.

Another former prime minister, David Cameron also expressed his concern about the plan.

"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,"Cameron, from Johnson's Conservative Party, told reporters on Monday.

"So I do have misgivings about what is being proposed."

UK Justice Secretary Robert Buckland said on BBC television that he would resign if the rule of law were broken in an "unacceptable" way.

'COME BACK FROM THE BRINK'

The main opposition Labour Party also said it will vote against Johnson's proposals as they currently stand, adding to the pressure on the parliamentary arithmetic for the prime minister.

Nevertheless, with a majority in the Commons of 80, it looks like Johnson will have enough votes to proceed with his plan.

The EU says if the bill is not effectively scrapped there will be no trade deal to cover everything from car parts to food.

"Come back from the brink, re-establish trust and keep your word," Ireland's European Affairs Thomas Byrne said.

If, as expected, it is passed in its second reading on Monday, there will be four more days of debate on the bill's fine print - lasting into Tuesday (Sept 22) next week.

After the vote on Monday, there will be further votes on attempts to change the wording and meaning of the law, and a final vote to decide whether it goes to the next stage.

The most crucial vote is likely to take place next week. If the bill then passes the lower house, it will undergo scrutiny in the House of Lords where opposition from Conservative members is expected to be even stronger.

EU leaders continued to express their concerns about the UK's actions.

Charles Michel, the president of the EU leaders' council, said Britain's international credibility is at stake.

Michel Barnier, the EU's chief Brexit negotiator, rejected the UK's claim that the divorce treaty is 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," Barnier tweeted.

Simon Coveney, Ireland's foreign minister, said on BBC television on Sunday that the UK had created "enormous tensions" in the trade negotiations with the EU.

But on Monday, Coveney said there are not that many outstanding issues in the way of securing a "thin" free-trade deal between the European Union and Britain, which is the best-case scenario at this stage .

"We need to also focus on how do we get this deal done... to deal with the outstanding issues. There aren't many of them. There are a few that are needed in terms of getting a basic and thin free-trade (deal), which is all that is possible at this stage to avoid tariffs and quotas," Coveney said in an interview with Newstalk Radio.

He said the key issues relate to Northern Ireland: state aid in the British region; the risk of goods leaking into the EU single market via Northern Ireland; and export declarations for the movement of goods from Northern Ireland to the rest of the United Kingdom.

Barnier and UK Brexit negotiator David Frost had a public Twitter spat on Sunday over the question of Northern Ireland, and whether the EU had threatened to block food imports from Britain into the province during the trade talks.

Barnier said the EU was not refusing to allow food imports across, whereas Frost said the threat had been made.

It's an argument the government continued to make on Monday, with Policing Minister Kit Malthouse telling BBC radio that the legislation was necessary to allow food to continue to pass from Great Britain to Northern Ireland.

"If we get to a situation where we're not recognised as a third country and it therefore becomes illegal to move food to Northern Ireland, what is a UK prime minister supposed to do?" Malthouse said.

"This is the solution that needs to be offered if we're going to have resolution. Northern Ireland is unequivocally part of the UK customs territory, so that fact that that is now being brought into question is a very difficult situation for us to face."