Britain and the United States will "probably end up" striking a bilateral trade deal, but agreement is "very unlikely" if the terms of the Ireland peace deal are broken, Nancy Pelosi said Friday.

"This is not said as any threat, it's a prediction, if there's destruction of the Good Friday accords, we're very unlikely to have a UK-US bilateral," the House of Representatives speaker said on a visit to London.

Britain is angling for a trade deal with Washington now it has left the European Union.

But it remains locked in talks with Brussels and Dublin about how best to implement tricky post-Brexit trade rules for the British province of Northern Ireland.

Nancy Pelosi, Speaker of the US House of Representatives, posed for media outside of 10 Downing Street on Thursday as she arrived for a meeting with Britain's Prime Minister Boris Johnson
Nancy Pelosi, Speaker of the US House of Representatives, posed for media outside of 10 Downing Street on Thursday as she arrived for a meeting with Britain's Prime Minister Boris Johnson AFP / JUSTIN TALLIS

The Northern Ireland Protocol aims to keep an open border between Northern Ireland and EU member state Ireland, a key plank of the 1998 Good Friday Agreement that brought an end to decades of violence over British rule.

But pro-British unionists say the deal to mandate checks on goods crossing the Irish Sea from mainland Britain creates another de facto border and puts Northern Ireland's place in the wider UK in jeopardy.

London and Brussels agreed earlier this month to indefinitely extend a grace period on implementing some checks -- a move welcomed by Pelosi.

"I'm so glad that more time has been given for the negotiations... because there has to be an agreement," she told the Chatham House international affairs think tank.

Opposition to the Northern Ireland Protocol has sparked some of the worst unrest in recent years
Opposition to the Northern Ireland Protocol has sparked some of the worst unrest in recent years AFP / PAUL FAITH

Any UK-US trade deal will need to be passed by the House of Representatives led by Pelosi, a Democrat whose boss, US President Joe Biden, is of Irish descent.

She stressed the Good Friday deal, brokered under US president Bill Clinton, is "very, very, very, very respected in the Congress of the United States.

"We probably will end up there," she said of a trade deal, "but we have a path that has to come though the EU, recognising the importance of the Good Friday accords".

Pelosi said she discussed the issue with Prime Minister Boris Johnson during a meeting at Downing Street on Thursday.

"He gave me some reading material. He may be coming to the US... soon and I told him I'd be reading what he gave me and asking some questions about it when we meet."

Despite the wranglings over Ireland and disagreements about the Afghanistan withdrawal, Pelosi played up the close ties between Britain and the US.

"Nobody has the special relationship that the UK and the US have with each other," she said.

"So may people say 'why we can't have the same relationship', so many countries say that, but it's historic, it's values, it's so many things."