Boris Johnson has been warned he cannot unilaterally set aside the Brexit Withdrawal Agreement, with the president of the European Commission warning it "cannot be unilaterally changed, disregarded, disapplied".
The prime minister has angered politicians both at home and abroad with his plan to implement the Internal Markets Bill - a piece of legislation that will override the Northern Ireland protocol contained within the Withdrawal Agreement.
Critics say the UK would be breaching international law if it reneges on the agreement.
Warnings have come from both the EU and US that they could not agree a free trade deal with the UK if it does not uphold the agreement.
EC President Ursula von der Leyen, in her annual State of the Union address, said when the Withdrawal Agreement was signed by the EU and UK in October, it was agreed to be the only way to guarantee the Northern Ireland peace process.
She said trust would be undermined if the UK started to go back on its international treaty obligations.
Addressing the Parliament in Brussels, Mrs von der Leyen said: "This Withdrawal Agreement took three years to negotiate and we worked relentlessly on it line-by-line, word-by-word, and together we succeeded.
"The European Union and the UK jointly agreed that it was the best and only way for ensuring peace on the island of Ireland and we will never backtrack on that.
"This agreement has been ratified by this house and the House of Commons. It cannot be unilaterally changed, disregarded, disapplied.
"This is a matter of law and trust and good faith."
Ms von der Leyen added that "very little time" remains for a trade deal between the EU and UK.
Mr Johnson has insisted the Internal Markets Bill is only intended to provide a legal "safety net" to protect the peace process and ensure the EU could not impose tariffs on goods moving to Northern Ireland from the rest of the UK.
MPs approved a second reading of the Bill on Monday and it is now in the committee stage - amendments will be voted on next week.
Northern Ireland Secretary Brandon Lewis has previously admitted it would break the law in a “very specific and limited way” but the justice secretary has said it does not, yet.
Robert Buckland told ITV News the introduction of the Bill does not break the law, but said it could if it is used in the future.
He said the Bill would only be used if negotiations with the EU on the future relationship fail to reach an agreement.
But senior politicians in the US have threatened to block any free trade deal between America and the UK if Mr Johnson goes back on his Brexit agreement with the EU.
Four congressmen have written to the prime minister, urging him to uphold Britain's commitments to Northern Ireland under the Withdrawal Agreement as part of Brexit.
The UK Internal Market Bill - which would override aspects of the Withdrawal Agreement - cleared its first Commons hurdle on Monday despite deep misgivings by some senior Tories, with amendments to the Northern Ireland provisions expected to emerge in the Commons next week.
The congressmen, led by chairman of the House Foreign Affairs Committee Eliot Engel, warned Mr Johnson the Government must respect its open border and peace process with Northern Ireland.
It comes as UK Foreign Secretary Dominic Raab has travelled to the US for talks with US Secretary of State Mike Pompeo on a range of issues.
The congressmen wrote: “Many in the United States and in Congress consider the issues of the Good Friday Agreement and a potential US-UK Free Trade Agreement inextricably linked.
“With the issues raised in this letter in mind, we therefore urge you to abandon any and all legally questionable and unfair efforts to flout the Northern Ireland protocol of the Withdrawal Agreement and look to ensure that Brexit negotiations do not undermine the decades of progress to bring peace to Northern Ireland and future options for the bilateral relationship between our two countries.”
They said they will not support a free trade agreement between the United States and the United Kingdom if Britain fails to uphold its commitments with Northern Ireland.
The letter, which was also signed by Mr Engel’s fellow Democrats Richard Neal and William Keating as well as Republican Peter King, comes less than a week after Nancy Pelosi said there would be “absolutely no chance” of a US/UK trade deal if Mr Johnson overrides the Brexit deal with Brussels.
The Speaker of the US House of Representatives Nancy Pelosi said the American Congress would never pass an economic agreement that it felt could “imperil” the Northern Ireland peace accord.
Mr Raab will also meet Ms Pelosi during his Washington visit, with Brexit expected to be one of the main items discussed.