Boris Johnson has urged MPs to support a bill which modifies the Brexit deal.
The PM said the Internal Markets Bill, would “ensure the integrity of the UK internal market” when the Brexit transition period ends.
He also claimed it would protect the Northern Ireland peace process and hand powers to Scotland.
Critics say the move will damage the UK’s international standing after a minister admitted the plans break international law.
The European Commission has called for an extraordinary meeting to discuss the bill – and the Scottish government has not ruled out legal action to prevent it becoming law.
European Commission Ursula von der Leyen said: “Very concerned about announcements from the British government on its intentions to breach the Withdrawal Agreement. This would break international law and undermines trust.”
The Internal Market Bill – which was introduced to Parliament on Wednesday – could override parts of the Withdrawal Agreement that secured the UK’s exit from the EU in January.
Ministers say it is needed to prevent “damaging” tariffs on goods travelling from the rest of the UK to Northern Ireland if negotiations with the EU on a free trade agreement fail.
‘Moral high ground’
But senior Conservatives have warned it risks undermining the UK’s reputation as an upholder of international law.
Tobias Ellwood, chairman of the Commons Defence Committee, said the UK would “lose the moral high ground” if the government went through with the changes.
Speaking at Prime Minister’s Questions, Mr Johnson said: “My job is to uphold the integrity of the UK but also to protect the Northern Ireland peace process and the Good Friday Agreement.
“And to do that, we need a legal safety net to protect our country against extreme or irrational interpretations of the Protocol, which could lead to a border down the Irish Sea, in a way that I believe would be prejudicial to the interests of the Good Friday Agreement and prejudicial to the interests of peace in our country. And that has to be our priority.”
Commenting on a similar argument by Health Secretary Matt Hancock, a former minister told the BBC: “I cannot allow anyone to get away with saying the government is doing this to protect the peace process. This does the precise opposite.
“It is about the internal market in the UK and is more likely to lead to a hard border [between Northern Ireland and the Republic of Ireland] which will imperil the peace process.”
‘Specific and limited’
On Tuesday, the permanent secretary to the Government Legal Department, Sir Jonathan Jones, has resigned from his role over concerns about the government breaching its obligations under international law.
And in the Commons, Northern Ireland Secretary Brandon Lewis admitted the bill would break international law in a “very specific and limited way”.
It would allow the UK government to “dis-apply” the EU legal concept of “direct effect” – which gives EU law supremacy over UK law in areas covered by the Withdrawal Agreement – in “certain, very tightly defined circumstances,” he told MPs.
The Scottish government, meanwhile, has said it will not consent to a change in the law along these lines, arguing that it would undermines devolution.
The bill has also been attacked by the Welsh Brexit minister, Labour’s Jeremy Miles, who accused the government of “stealing powers from devolved administrations”.
Cross border trade
The legislation will see Scotland, Wales and Northern Ireland handed powers in areas such as air quality and building efficiency currently regulated at EU level.
It will also set up a new body – the Office for the Internal Market – to make sure standards adopted in different parts of the UK do not undermine cross-border trade.
The new body will be able to issue non-binding recommendations to the UK Parliament and devolved administrations when clashes emerge.
However, plans to hand UK ministers extra powers to ensure the application of customs and trade rules in Northern Ireland have prompted a row over the UK’s legal obligations in its exit deal.
Under the UK’s withdrawal agreement, Northern Ireland is due to stay part of the EU’s single market for goods in a bid to avoid creating a hard border with the Irish Republic.
In parallel with talks over a post-Brexit trade deal, the UK and EU are negotiating the precise nature of new customs checks that will be required.