US presidential candidate Joe Biden has said he will not allow the Good Friday Agreement to become a “casualty” of Brexit if he is elected in November.

The Democratic candidate said any trade deal between the US and UK had to be “contingent” on respect for the Northern Ireland peace deal.

On Wednesday night, he 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 foreign secretary Dominic Raab was visiting Washington DC in an attempt to reassure US politicians that Britain’s support for the Northern Ireland peace agreement was “absolute”.

Conor Burns, Tory MP for Bournemouth West, mocked Biden’s tweet and implied the US Democratic candidate did not properly understand the Good Friday Agreement.

“It is also called the Belfast Agreement so it doesn’t offend both traditions. Did you actually know that?” he wrote.

“Here if you need help.”

But shadow foreign secretary Lisa Nandy said Biden’s comments showed “the scale of damage” the UK government had done to Britain’s “standing” in the world”.

“They’ve lost trust and undermined co-operation at the moment we most need it – and all to tear up an agreement they negotiated,” she tweeted.

“Reckless, incompetent and utterly self-defeating.”

Boris Johnson has faced accusations of breaking international law by attempting to change parts of the Brexit Withdrawal Agreement made with Brussels, raising concerns that a hard border will be imposed on Ireland.

On Wednesday one of Johnson’s most senior law officers, Lord Keen, quit in protest at the government’s plans while delivering a withering verdict on the legality of the government’s policy in his resignation letter. 

“I have found it increasingly difficult to reconcile what I consider to be my obligations as a Law Officer with your policy intentions,” he wrote.

In the US, leading members of the Democratic Party – who currently hold the majority in the House – have warned any undercutting of the Good Friday Agreement could see them veto the US-UK trade deal currently being negotiated.

Speaker of the House Nancy Pelosi said there will be “absolutely no chance of a US-UK trade agreement passing the Congress” if the UK “violates its international agreements” and Brexit “undermines the Good Friday accord”.

Raab insisted the threat came from Brussels’ “politicisation” of Northern Ireland issues and told Pelosi the UK would never erect a hard border in Northern Ireland.

“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.”

But US Secretary of State Mike Pompeo defended the UK government, telling reporters in a press conference he was “confident they’ll get it right” and described the ‘special relationship’ as “stronger than ever”.

“We’ve made clear our view of the importance of the Good Friday Agreement, we know the complexity of the situation and we’ve done what we can to provide assistance where we can,” he said.

“In the end this will be a set of decisions with respect to this that the United Kingdom makes and I have great confidence that they will get this right,” he added.