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"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," Pelosi said.
"The Good Friday Agreement is treasured by the American people and will be proudly defended in the United States Congress," she said.
European diplomats said Britain was playing a game of Brexit chicken by threatening to wreck the process and challenging Brussels to change course.
Some fear Johnson may view a no-deal exit as a useful distraction from the coronavirus crisis.
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Irish Prime Minister Micheal Martin called on Britain to drop its plan to breach the divorce deal, telling the Financial Times it was not clear now if Johnson wanted a trade deal.
"The British government needs to move to restore trust and to give meaningful reassurance to the European negotiators," Martin said in an interview with the newspaper.
"Our colleagues in Europe, in particular those conducting the negotiations, are now wondering whether the will is there or not to arrive at a conclusion and get an agreement — and that is a very serious issue."
(Writer: Guy Faulconbridge | Editors: Kim Coghill, Peter Graff)
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