Nancy Pelosi has warned British authorities there will be no US-UK trade deal post-Brexit if there is any damage to the peace process in Ireland.
The Speaker of the US House of Representatives emphasised the US was a “guarantor” of the Good Friday Agreement.
She said her views and those of a US congressional delegation were made known at the “highest level” during a visit to London this week.
“Everyone we met with, we made clear. The Good Friday [Agreement] accords are not just a peace agreement, that would be justified enough for us to want to work very hard to protect. But it is something so elevated because of the numbers of hundreds of years of conflict that it resolved.”
The US had a “vested” interest in Irish peace, Ms Pelosi said, adding:
We made clear to them that while we want the best possible outcome, the people have spoken, they want to leave the EU, that's a decision their country is making and has made and as they work that out, not to think for one minute that there is any comfort for them in the fact that if they leave the EU, they would quickly have a US-UK trade agreement.
"That's just not on the cards if there's any harm done to the Good Friday accords. Don't even think about that."
Westminster has failed to agree with voters' EU exit, risking the threat of a no-deal Brexit and possible fresh violence in the North.
Ms Pelosi was speaking at a lunch hosted by Tánaiste Simon Coveney at Iveagh House.
Mr Coveney said Ireland must ensure the “troubled and tragic past must not come back to haunt us”.
Trade between Ireland and the US was worth half a trillion dollars, he said, and Washington had put over €540m in peace funds into helping transform the island of Ireland.
Ms Pelosi also met Taoiseach Leo Varadkar last night at government buildings and will today make a special address in Leinster House, before visiting the border region tomorrow.