Biden threatens UK trade deal over Brexit shambles

"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," he added.

Biden spoke out during a visit by British foreign secretary Dominic Raab to Washington.

The Democrats' concern comes after the UK threatened to renege on its EU withdrawal agreement, risking the re-imposition of a hard border between Ireland and Northern Ireland in violation of the Good Friday accord

But for his part, the British top diplomat blamed the EU for the situation when he met US secretary of state Mike Pompeo the same day.

"The threat to the Good Friday Agreement ... has come from the EU's politicisation of the issue," Raab said.

"What we cannot have is the EU seeking to erect a regulatory border down the Irish Sea between Northern Ireland and Britain," he added, even though his own government had agreed to the withdrawal plan earlier.

"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," Pompeo 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.

The US trade secretary, Robert Lighthizer, also said he believed the US and UK could clinch a "win-win deal" on trade.

Britain used to trade with America under the auspices of EU-brokered trade arrangements.

And the prospect of securing more favourable terms after Brexit on the basis of the UK-US 'special relationship' was a big promise by Raab's government.

But Nancy Pelosi, the Democratic speaker of the Democrat-controlled House of Representatives, which must pass Raab's transatlantic trade pact, echoed Biden's warning.

"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," she said in a statement after also having met Raab in the US capital on Wednesday.

"The Good Friday Agreement is the bedrock of peace in Northern Ireland and a beacon of hope for peace-loving people throughout the whole world," Pelosi said.

About 33 million Americans identify as being Irish, in a significant constituency ahead of the US elections in November.

And four congressmen, three Democrats and one Republican, who chair House committees, said the same as Biden and Pelosi in a joint letter on Tuesday, indicating broader support for the hardline stance.

"We ... urge you to abandon any and all legally questionable and unfair efforts to flout the Northern Ireland protocol of the [EU] withdrawal agreement," they said.