Britain headed into emergency talks with the European Union on Thursday facing warnings of legal action over a new Brexit bill and a threatening reminder of its obligations to Northern Ireland from leading US Democrat Nancy Pelosi.

Warnings redoubled too within the ranks of the ruling Conservative party as former prime minister John Major, who helped lay the foundations for Northern Irish peace in the 1990s, said his successor Boris Johnson risked trashing the UK's global reputation.

Britain's signature on any treaty or agreement has always been "sacrosanct", Major said, after ministers in London conceded that the new bill would breach an EU withdrawal treaty in the countdown to a full Brexit divorce.

"If we lose our reputation for honouring the promises we make, we will have lost something beyond price that may never be regained," he added.

European Commission vice president Maros Sefcovic headed to London demanding "clarifications" over the new UK Internal Market Bill, after it was submitted to a stormy session of parliament Wednesday.

"I came here to express serious concerns the European Union has over the proposed bill. So that will be the nature of our discussions today," Sefcovic told reporters Thursday prior to meeting his counterpart on a UK-EU joint committee, Michael Gove.

The bill would give British ministers unilateral powers to regulate trade among England, Scotland, Wales and Northern Ireland, once the force of EU law expires after a post-Brexit transition period at the end of this year.

But under the EU withdrawal treaty, Britain is meant to liaise with Brussels on arrangements for Northern Ireland, which will have the UK's only land border with the EU, and where 30 years of bloodshed were ended with a landmark peace deal in 1998.

Rushed through?

EU diplomats -- and Johnson's many critics at home including in the UK's devolved governments -- have ridiculed Downing Street's argument that the EU treaty was written "at pace" and contained unforeseen problems relating to Northern Ireland.

Sefcovic and Gove were to sit down on the last day of a parallel track of negotiations this week in London that have struggled to make headway on a future trading relationship as Britain unwinds nearly 50 years of European integration.

Johnson's critics say the new bill is aimed partly at torpedoing that track, so Britain can go its own way and forge other trade pacts free of EU oversight, not least with the United States.

However, House of Representatives Speaker Pelosi gave short shrift to any British hopes of Congress ratifying a future trade deal if it ploughs ahead with the new Brexit bill.

In a statement, she said London must respect the EU treaty's Northern Ireland Protocol, which envisages borderless trade with EU member Ireland as a way of upholding the 1998 peace pact.

"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 warned.

"The Good Friday Agreement is treasured by the American people and will be proudly defended in the United States Congress."

'Gross irresponsibility'

More immediately, the EU's executive commission circulated a paper setting out legal options against London including recourse to the European Court of Justice -- the supreme arbiter of EU law which Brexit, ironically, is meant to escape.

"A breach of the obligations under the Withdrawal Agreement would open the way to the legal remedies," a draft prepared by EU ambassadors and seen by AFP said.

EU diplomats said to avoid that, and the possibility of hefty fines against Britain, much hinged on the Sefcovic-Gove committee finding a way out.

Johnson spoke by phone on Wednesday evening with his Irish counterpart, Micheal Martin, who was left unimpressed by British assurances that the internal market bill is aimed in fact at preserving peace in Northern Ireland.

"I pointed out very strongly to him that this was very unsettling for Northern Ireland, that it was dragging Northern Ireland back into the centre stage," Martin told RTE radio, warning also that Johnson had eroded trust with the EU.

Looking at the broader track, the Irish premier added that British failure to compromise with the EU on a future trading regime would be "gross irresponsibility".

"A no deal will damage people's lives and economic prospects, very much indeed in addition to Covid-19," Martin said.