US presidential election frontrunner Joe Biden has insisted the Good Friday peace deal in Northern Ireland cannot become a "casualty" of Brexit.
The intervention by the Democratic Party nominee for the White House came as Prime Minister Boris Johnson faces concerted opposition to Government moves that would override the divorce deal with Brussels regarding trade with Northern Ireland.
As Foreign Secretary Dominic Raab was visiting Washington DC to discuss the issue, former vice president Mr Biden, who is leading incumbent president Donald Trump in a series of nationwide polls ahead of the November election, insisted a future trade deal between the US and UK could only happen if the peace agreement was respected.
Mr Biden tweeted: "We can't allow the Good Friday Agreement that brought peace to Northern Ireland to become a casualty of Brexit.
"Any trade deal between the US and UK must be contingent upon respect for the Agreement and preventing the return of a hard border. Period."
His comments came as the Prime Minister saw the resignation of a senior law officer, Lord Keen, as he was forced into a compromise over controversial plans to break international law by overriding the Brexit Withdrawal Agreement (WA).
The remarks echoed those of Democratic Speaker of the US House of Representatives Nancy Pelosi.
Before a meeting with Ms Pelosi, Mr Raab accused Brussels of the "politicisation" of Northern Ireland issues in the context of Brexit trade talks.
He said the EU stance threatened the Good Friday Agreement.
Mr Raab said he had made clear the UK has an "absolute" commitment to the Good Friday Agreement.
"The UK action here is defensive in relation to what the EU is doing, it is precautionary, we haven't done any of this yet, and it is proportionate," he said.
"What we cannot have is the EU seeking to erect a regulatory border down the Irish Sea between Northern Ireland and Britain."
Ms Pelosi has warned Congress would never pass a free trade agreement with the UK if legislation to override the Brexit divorce settlement was to "imperil" the peace process.
She said in a statement following her meeting with Mr Raab that she "welcomed his assurances but reiterated the same message that we delivered to the leaders of the UK in London last year: if the UK violates its international agreements and Brexit undermines the Good Friday accord, there will be absolutely no chance of a US-UK trade agreement passing the Congress".
Speaking to reporters after his meeting with Mr Raab, US Secretary of State Mike Pompeo said he had "great confidence" the British Government "will get this right in a way that treats everyone fairly and gets a good outcome for what it was the people of the United Kingdom voted for now several years back".
The Government will table an amendment to the UK Internal Market Bill, giving MPs a vote before it can use powers which would breach the deal brokered with Brussels last year.
Around 30 Tory rebels were thought to be preparing to vote for an amendment on Tuesday which would have required a Commons vote before the provisions in the Bill relating to Northern Ireland could come into force.
Downing Street relented and announced in a joint statement with Conservative MPs Sir Bob Neill and Damian Green that it would seek to amend the Bill to require the Commons to vote before a minister can use the powers contained within it.
The statement said: "Following constructive talks over the last few days, the Government has agreed to table an amendment for Committee Stage.
"This amendment will require the House of Commons to vote for a motion before a minister can use the 'notwithstanding' powers contained in the UK Internal Market Bill."
But Mr Johnson risked further conflict with the EU when he said Brussels was not acting in good faith.
He told the Sun: "We assumed our EU friends and partners would want to negotiate in good faith. We've been paid up members for 45 years."
The Government's top law officer for Scotland resigned on Wednesday amid reports he was unhappy about the plans to override the Withdrawal Agreement.
Lord Keen of Elie QC, the Advocate General, said in his resignation letter: "Over the past week I have found it increasingly difficult to reconcile what I consider to be my obligations as a Law Officer with your policy intentions with respect to the UKIM Bill.
"I have endeavoured to identify a respectable argument for the provisions at clauses 42 to 45 of the Bill but it is now clear that this will not meet your policy intentions.
Labour's shadow attorney general Lord Falconer said: "This has been a week of chaos from the Government's own law officers, whose legal advice has been renounced by its own Government and the voice of the law officers has been muted, and their authority is completely shot."