A US-UK trade agreement will not come through Congress if Brexit undermines the Long Friday Agreement, Speaker of the US House of Representatives have said.
Democrat Nancy Pelosi, whose party controls the House, said Britain's exit from the EU could not be allowed to jeopardize the Irish peace treaties.
Her comments came after the US national security adviser said the UK would be "first in line" for a trade agreement.
John Bolton spoke after meeting Prime Minister Boris Johnson in London.
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The reintroduction of border controls between Northern Ireland and the Republic of Ireland on Britain leaves the EU without a mutual agreement on 31
"Whatever form it takes, Brexit cannot be allowed to jeopardize the good Friday agreement, including the seamless border between Ireland and Northern Ireland," Pelosi said in a statement on Wednesday.
"Tough old bargain"
On Tuesday, Mr Bolton said the Trump administration supported a non-Brexit deal and added Washington would propose a faster series of trade deals in the event of one.
He said these could be done on a "sector by sector" basis, with a manufacturing agreement first. A trade agreement for financial services and agriculture would not be the first thing agreed, he added.
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Asked if his proposed plan would follow the WTO rules, Bolton said "our trade negotiators seem to think it is."
He said there would be enthusiastic bipartisan support in Congress for rapid ratification at every step.
Mr Johnson said there were "all kinds of" opportunities for British business in the United States, especially service companies, but the negotiations will be a "tough old bargain".
However, critics warn that the UK must give in to some US demands in return for each trade agreement.
Former UK Foreign Minister Jack Straw, who served under a Labor government, described Bolton as "dangerously bellicose".
He suggested that the United Kingdom should accept certain US requirements, for example, to allow the import of American chlorine-washed chicken.
"This is a very transaction administration … you get nothing for nothing," he told BBC Radio 4's Today program.
Lewis Lukens, a former Deputy Head of Mission at the US Embassy in London and a former US Ambassador, said that Bolton aligned with President Trump's "America's first agenda" and would place "strong demands" on the UK to back the US position on issues such as China, Iran and Chinese technology giant Huawei.
Mr Johnson is expected to have his first face-to-face meeting as prime minister with Trump later this month at the G7 summit in France.