Nancy Pelosi has warned the UK there will be "absolutely no chance" of a trade deal with Washington passing Congress should the government override the Brexit withdrawal agreement signed by Boris Johnson.
The Democratic speaker of the House of Representatives said Brexit could not be allowed to “imperil” the Good Friday Agreement - brokered to establish peace in Northern Ireland after years of sectarian conflict.
It comes after Mr Johnson unveiled plans to override key elements of the Brexit deal with the EU regarding Northern Ireland.
In a statement on Wednesday Ms Pelosi said: "The Good Friday Agreement is the bedrock of peace in Northern Ireland and an inspiration for the whole world.
"Whatever form it takes, Brexit cannot be allowed to imperil the Good Friday Agreement, including the stability brought by the invisible and frictionless border between the Irish Republic and Northern Ireland.
"The UK must respect the Northern Ireland Protocol as signed with the EU to ensure the free flow of goods across the border.
"If the UK violates that international treaty and Brexit undermines the Good Friday accord, there will be absolutely no chance of a US-UK trade agreement passing the Congress.
"The Good Friday Agreement is treasured by the American people and will be proudly defended in the United States Congress."
While the balance of power in the House remains tipped in the favour of the Democratic party, the approval of Ms Pelosi and her majority will be vital in securing any trade agreement between the two nations.
Speaking on Newsnight on Wednesday evening, one of the most senior Democratic congressmen, Richard Neal, echoed Ms Pelosi’s words.
"There will be no trade agreement on a bilateral basis with the UK, if they re-establish a hard border.
“This idea you could arbitrarily determine that you could re-establish the border after giving repeated assurances that that would not happen is a violation of the good faith that we all entered into…in this remarkable achievement called the Good Friday Agreement.”
The UK government is pushing forward with a new internal market bill, which will allow ministers to override its treaty agreement with the EU by unilaterally waiving customs documents on goods travelling from Northern Ireland to the British mainland and tariffs on exports travelling the other way.
The policy also limits the EU’s ability to curtail the use of state aid subsidies. Talking through the legislation with the Commons on Tuesday, Northern Ireland secretary Brandon Lewis said the bill would “break international law in a very specific and limited way”.
However, the government has argued the legislation is a necessary step towards protecting the Northern Ireland peace process in a situation where London and Brussels are unable to hash out a free trade deal before the transition period runs out.
Watch - Yahoo UK’s Finance Editor, Edmund Heaphy explain what no-deal Brexit is