(Adds background on Good Friday accord, Bolton quote)
WASHINGTON, Aug 14 (Reuters) - There is no chance the U.S. Congress would approve a planned U.S.-British trade deal if Britain's exit from the European Union undermines the landmark 1998 Good Friday peace agreement for Northern Ireland, House of Representatives Speaker Nancy Pelosi said on Wednesday.
U.S. President Donald Trump's administration is negotiating a free trade deal with Britain that would go into effect after it completes Brexit, now scheduled for Oct. 31. Any U.S.-British trade agreement would have to pass the U.S. Congress.
The Good Friday agreement, which helped end three decades of violence in Northern Ireland, dismantled all physical border infrastructure between EU member Ireland and Northern Ireland, a province of Britain, guaranteeing that people and goods on either side can move freely.
"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," Pelosi, the top Democrat in Congress, said in a statement.
"If Brexit undermines the Good Friday accord, there will be no chance of a U.S.-UK trade agreement passing the Congress," Pelosi added.
Congress is split between Democrats, who control the House, and Trump's fellow Republicans, who control the Senate.
An impasse with the EU leaves Britain facing the possibility of an exit without any formal transition period or legal agreement covering issues such as trade, data transfers and border policy.
U.S. national security adviser John Bolton, visiting London this week, said Washington would enthusiastically support a no-deal Brexit if that is what Britain's government decided to do, and that a trade deal with the United States would help cushion the blow of Britain's departure.
During a visit aimed at reassuring Britain over British-U.S. ties, Bolton told British Prime Minister Boris Johnson that Trump wants to see a successful Brexit and that Washington would be ready to work quickly on a trade pact.
Many in Ireland believe a no-deal Brexit would be catastrophic for the Good Friday agreement.
Of particular concern is what to do about the so-called backstop insurance policy in the previous deal negotiated by Johnson's predecessor as prime minister, Theresa May, to prevent the return to a hard border between Northern Ireland and Ireland.
May's proposal, rejected three times by the British parliament, would force Britain to obey some EU rules if no other way could be found to keep the land border open.
Bolton said during his London visit there was "zero chance" that a no-deal Brexit would undermine the accord.
(Reporting by Susan Heavey; Editing by Sonya Hepinstall and Will Dunham)