A US Congressman has warned that there will be no US-UK trade deal if Boris Johnson's government proceeds with the Internal Market Bill.
It comes as key parts of Boris Johnson's controversial Brexit legislation were rejected by the Lords.
Peers moved to strip controversial clauses from the UK Internal Market Bill that would enable ministers to break international laws.
The Lords voted 433 to 165, majority 268, to reject law-breaking powers after fierce criticism by Tory former leader Michael Howard and Tory ex-chancellor Ken Clarke.
The Government immediately responded by insisting it would not back down.
Brendan Boyle, whose father is from Co Donegal and who has been tipped as a candidate to be US president-elect Joe Biden's Irish ambassador, said: "This is a clear red line for us."
Asked by Channel 4 News if there was any hope for a trade deal if the powers in the Bill were not scrapped, Mr Boyle responded: "As we've discussed before, this is a clear red line for us, from president-elect Biden and Speaker (Nancy) Pelosi, to me and many of my colleagues - including on both sides of the aisle. If the UK moves forward with this Internal Market Bill, and decides to essentially rip up the Withdrawal Agreement that they negotiated and signed less than a year ago, then there will be no US-UK trade deal. Period."
Mr Biden warned during his successful campaign that a trade deal with the US is "contingent" on the prevention of a return to a hard border on the island of Ireland.
Earlier, EU chief negotiator Michel Barnier was in London to resume trade talks with Downing Street counterpart Lord Frost. He tweeted that he was "happy to be back" in the capital, with the two teams "redoubling our efforts" for a deal.
Despite the pressure on the Government, Environment Secretary George Eustice said it would reinstate the controversial clauses.
But he sought to reassure Mr Biden that there would be no need for a hard border. "All of that work is being done and because that work is being done there will be no need for checks on the Northern Ireland border," he told BBC Radio 4's Today programme.
There have been suggestions that No 10 could avoid a clash with the White House by leaving the legislation if a trade deal is brokered with the EU.
Lord Dodds told peers last night that many unionists feel "deeply frustrated" about the proposed future trading arrangements. The DUP's former Westminster leader told the Lords it was "unacceptable to create barriers between Northern Ireland and the rest of the United Kingdom".
He added: "We must come to sensible, pragmatic arrangements."