The US House of Representatives Speaker has warned lawmakers will block a trade pact with Britain if its exit from the EU undermines Northern Ireland's peace accord.

President Donald Trump and senior advisors to Britain's new Prime Minister Boris Johnson have spoken recently of fast-tracking a bilateral trade deal once Brexit is complete.

But any such agreement would need the green light from Congress, and House Speaker Nancy Pelosi - President Trump's political nemesis on Capitol Hill - made clear she would not play ball if the Irish peace deal were put at risk.

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Northern Ireland is part of the United Kingdom and is thus set to leave the European Union while the neighboring Republic of Ireland is a separate EU member state.

Reimposing controls along their shared border if Britain leaves without a deal - a so-called "hard Brexit" - would put the 1998 peace deal in jeopardy.

"Whatever form it takes, Brexit cannot be allowed to imperil the Good Friday Agreement, including the seamless border between the Irish Republic and Northern Ireland," Ms Pelosi said in a statement.

"If Brexit undermines the Good Friday accord, there will be no chance of a US-UK trade agreement passing the Congress."

The 1998 Good Friday Agreement brought the decades-old Northern Ireland conflict to an end. But how to handle Northern Ireland has emerged as a core issue for Brexit negotiators.

Critics have warned that Brexit might require reimposing a hard border on the island, a move that would essentially upend the agreement that has kept peace in Northern Ireland for the past two decades.

Goods and people freely cross the border, as both countries are currently members of the EU, and the withdrawal agreement negotiated last year between London and Brussels contains a "backstop" plan to maintain this situation whatever happens with Brexit.

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However, British MPs have rejected it three times and Johnson warns the backstop must go or Britain will leave the EU on October 31 without any deal.

Ms Pelosi, strongly signaled that Republicans would join her Democrats in opposing a trade pact if Brexit undermines the peace deal.

"The peace of the Good Friday Agreement is treasured by the American people and will be fiercely defended on a bicameral and bipartisan basis in the United States Congress," she said.

The Republican co-chair of the Friends of Ireland group in the US Congress, Pete King, reportedly said jeopardizing the open border was a "needless provocation" over which his party would have no hesitation defying Trump.

#Brexit cannot be allowed to imperil the Good Friday Agreement. The peace of the Good Friday Agreement is treasured by the American people and will be fiercely defended on a bicameral and bipartisan basis in the US Congress. https://t.co/zfRpcGnWXv — Nancy Pelosi (@SpeakerPelosi) August 14, 2019

Those in Congress with a strong belief in Northern Ireland and the Good Friday agreement "would certainly be willing to go against the president," Mr King told The Guardian.

After his first phone call with the new British leader late last month, President Trump said talks on a "very substantial" post-Brexit trade deal were already underway.

Last week Johnson dispatched top aides including Foreign Secretary Dominic Raab to Washington in a bid to fast-track the negotiations.

And on Monday US National Security Advisor John Bolton, a hawkish Trump aide, said Washington wanted to "move very quickly" on the trade pact after Britain exits the EU.

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House Democrat Brendan Boyle called Bolton's trade talk "nonsensical" and said the comments should not be taken seriously. Mr Boyle added on Twitter that he "strongly" supports Ms Pelosi's position.

Any US trade deal needs final approval from the US Congress, where political power is split: Pelosi's Democrats control the House of Representatives, while the Senate is led by Republicans.

Dozens of US lawmakers claim Irish ancestry, and the Friends of Ireland caucus in Congress has long advocated for peace and justice in Ireland and Northern Ireland.

George Mitchell, a former US Senate majority leader with Irish roots, was president Bill Clinton's envoy to Northern Ireland and led the all-party peace negotiations in the 1990s.