Joe Biden has crashed into the row over Boris Johnson's law-breaking Brexit plan - and warned it could scupper a US-UK trade deal.
The Presidential frontrunner joint mounting anger in the US Congress as he warned he would not let Northern Irish peace become a "casualty of Brexit".
Tory ministers were scrambling to downplay the warning the morning, insisting their plan would not breach the 1998 Good Friday Agreement.
But the Democratic candidate's intervention piles pressure on Boris Johnson, despite the Prime Minister agreeing a climbdown last night.
It also comes after Lord Keen, the Advocate General for Scotland, became the second top legal advisor to quit the government in protest. © PAIt also comes after Lord Keen, the Advocate General for Scotland, became the second top legal advisor to quit the government
Last night Mr Johnson struck a deal with backbench Tories to see off a revolt on the Internal Market Bill.
The Bill would give UK ministers power to override parts of last year's Brexit deal, to decide which goods carry tariffs when moved from London to Belfast.
That is a clear breach of the Withdrawal Agreement, which said the EU and UK should decide jointly.
Under the compromise, the PM will still be able to break international law - but he'll have to get formal approval from parliament first.
Ministers would have to table an amendment requiring the Commons to vote before the contentious powers could be used.
Another amendment limiting the scope of judicial review will also be tabled. © AFP via Getty ImagesLast night Boris Johnson struck a deal with backbench Tories to see off a revolt on the Internal Market Bill
But the compromise did not prevent the resignation of Lord Keen - who left with a stinging letter.
He told the PM: "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.
"In these circumstances I consider that it is my duty to tender my resignation from your Government."
Last night Mr Biden said any UK-US trade deal had to be "contingent" on respect for the Good Friday Agreement. © REUTERSBiden said any UK-US trade deal had to be
The row could prove crucial if Mr Biden formally replaces Donald Trump as President in January.
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."
The remarks echoed those of Democratic Speaker of the US House of Representatives Nancy Pelosi.
Tories tried to minimise Mr Biden's warnings.
Health Minister Edward Argar told LBC that there was "no way this government will put the Good Friday Agreement at risk".
He added: "Actually we're working hard to protect the Good Friday Agreement to make sure we deliver on the will of the people at the end of the transition period."
But other Tories failed to strike the same diplomatic tone, with US-born Tory MP Joy Morrisey accusing the former Vice President of "sounding off" in a now deleted tweet.
She told Mr Biden to "climb down off your moral high horse and maybe fact check the situation".
In a second tweet she accused Mr Biden of intervening to win the Irish American vote in the November election.
Foreign Secretary Dominic Raab visited Washington DC to discuss the issue this week.
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".
Video: Total failure of social distancing during House of Commons vote, claims MP (PA Media)