The leader of the US House of Representatives said it was “deeply concerning” the British government was planning to unilaterally end protocol checks previously agreed with the EU.
Ms Pelosi said she had previously told Mr Johnson and foreign secretary Liz Truss that if they choose to “undermine” the Good Friday Agreement, then Congress “cannot and will not support a bilateral free trade agreement with the UK”.
Mr Johnson and Ms Truss have insisted they do not plan to tear up the protocol completely, but aim to unilaterally “fix” the agreement through new legislation to override parts of the Brexit deal.
However, the powerful US figure issued a stern rebuke. She said that maintaining arrangements designed to ensure there is no hard border in Ireland was “absolutely necessary” to uphold Good Friday Agreement.
“It is deeply concerning that the UK is now seeking to unilaterally discard the Northern Ireland Protocol,” Ms Pelosi said in a statement issued on Thursday.
“Negotiated agreements like the protocol preserve the important progress and stability forged by the Good Friday Accords, which continue to enjoy strong bipartisan and bicameral support in the US Congress,” Ms Pelosi added.
The US speaker urged Mr Johnson’s government to resume “constructive, collaborative and good-faith negotiations” with the EU to implement “an agreement that upholds peace”.
US president Joe Biden downplayed the prospect of a trade deal with the UK when he met Mr Johnson in September, when the prime minister was forced to admit: “Joe has a lot of fish to fry.”
His administration is understood to be reluctant to enter talks on a comprehensive post-Brexit deal because of his concerns about the ongoing row with the EU over Northern Ireland.
Ms Truss announced earlier this week that legislation would create new “green channel” lanes for goods travelling from Great Britain to Northern Ireland. Only goods destined for the Republic of Ireland would be subject to customs checks and other red tape.
The DUP is currently blocking the re-establishment of Stormont’s institutions in protest at the protocol, with Irish nationalist Sinn Fein urging the unionist party to restart power-sharing arrangements.
It comes as Joao Vale de Almeida, the EU ambassador to the UK, said no “credible alternative” to the protocol had been identified – as he warned that unilateral UK action to override it could “nix” the agreement altogether.
He revealed that EU Commission negotiators have been given a firm instruction from the 27 member states that there can be no change in their mandate, and no renegotiation.
No 10 is calling for more flexibility from European Commission negotiator Maros Sefcovic over British demands to cut checks on goods, with a vow to introduce the oveeride legislation soon if UK demands are not met.
Speaking at a Westminster lunch today, Mr Vale de Almeida said: “It’s not very reassuring to go into a negotiation where you are presented with two options – either negotiation or unilateral action to override the treaty.”
Echoing Mr Johnson’s claim that he wanted to “fix not nix” the protocol, the ambassador said: “This is not the best way to fix, this is rather a way maybe to nix.
Mr Vale de Almeida added: “If we want to fix it – which is what we want, and I understand that that is what the government wants as well – we need to create a better atmosphere.”