Biden shared a strongly-worded letter signed by four senior members of Congress, which urged Johnson to “abandon any and all legally questionable and unfair efforts” to breach its agreement with the EU.

“The United States Congress will not support any free trade agreement between the United States and the United Kingdom if the United Kingdom fails to preserve the gains of the Good Friday Agreement and broader peace process,” the letter said. “If these reported plans were to go forward, it would be difficult to see how these conditions could be met.”

An open border was a key requirement of the 1998 Good Friday Agreement that brought an end to more than 30 years of violence over British rule in Northern Ireland.

Nevertheless, at a press conference with Britain’s foreign secretary, Dominic Raab, on Wednesday in Washington, US Secretary of State Mike Pompeo said trade talks with the UK were progressing well. He also said he backed the UK as a trustworthy partner, despite the row over Johnson’s plan to rip up the Brexit deal.

“I am confident they’ll get it right,” Pompeo said. “We know the complexity of the situation.”

The British government hasn’t put a firm timetable on securing a free-trade agreement with the US, which has been touted as a key post-Brexit prize. The US is already the largest destination for British exports outside the EU, and trade between the two countries amounted to US$270 billion in 2019. A free-trade agreement with the US would be expected to boost Britain’s GDP by 0.16 per cent over the long run, according to a UK government estimate published in March.

Raab also met top Democrat Nancy Pelosi, who last week warned there was “absolutely no chance” of a UK-US trade deal if the move causes Brexit to undermine the Good Friday Agreement.

But Raab said the measures contemplated by Britain were “defensive” against the EU.

“It is precautionary; we haven’t done any of this yet. And it is proportionate,” he said.

“Our commitment to the Good Friday Agreement and to avoid any extra infrastructure at the border between the north and the south is absolute,” he said.

Johnson on Wednesday said he had full confidence that Britain and the EU would avoid a potentially disastrous cliff-edge “no deal” at the end of this year.

But he refused to back down on controversial new legislation that he openly admits will break international law.

Johnson told MPs that a “no deal” was “not what this country wants and it’s not what our EU friends and partners want from us”.

“Therefore I have every hope and expectation that will not be the outcome,” he told a parliamentary committee during more than two hours of questioning.

The UK Internal Market Bill was put before parliament this week, despite EU calls for it to be withdrawn and stark reminders of the need to uphold treaty obligations.

Johnson has claimed the EU could “blockade” food and agricultural products heading to Northern Ireland from mainland Britain by imposing higher duties and tariffs.

Northern Ireland will have Britain’s only land border with the EU from January 1, and remains bound by some EU rules to ensure its border with Ireland stays open.

Johnson said EU officials had “signally failed” to rule out preventing goods reaching Northern Ireland from Britain if no trade agreement was made.

EU chief executive Ursula von der Leyen earlier said the divorce deal that allowed Britain to leave the bloc could not be “unilaterally changed, disregarded or misapplied”.

“This is a matter of law and trust and good faith,” the European Commission president said in a speech, warning the chances of securing a trade deal were fading.

EU leaders have dismissed Johnson’s claims as “spin”, while the ramifications of the government’s admission the bill would break the law rumbled on.

Several of Johnson’s own Conservative MPs have spoken of their unease, while every living former prime minister has warned of the risk to Britain’s global reputation.

In another blow for the government, its most senior law officer for Scotland, Richard Keen, quit amid reports he opposed the plans to flout the Brexit Withdrawal Agreement.

“Lord Keen has resigned as Advocate General for Scotland. The prime minister thanks him for his service,” a short statement from Downing Street read.

Further votes on the bill are expected in parliament next week, but the government agreed to a compromise with Tory rebels.

It said it would allow MPs a vote before using powers which would break international law, according to a joint statement from Johnson’s office, and two senior Conservatives.

Bloomberg and Agence France-Presse