Britain's government on Friday vowed to press on with troubled EU talks, despite rejecting an ultimatum to withdraw Brexit legislation that has sparked a rebellion from outraged Conservatives

in its own ranks.

Prime Minister Boris Johnson trumpeted a win by announcing a free-trade agreement with Japan -- its first such pact since Brexit took effect in January.

But the far greater prize of an FTA with the European Union remains elusive.

"It is clear that anyone has to stick to what he already agreed to. Treaties have to be obeyed," German Finance Minister Olaf Scholz told reporters in Berlin.

The EU, warning that the contentious UK bill has "seriously damaged trust" between the two sides, said it would take Britain to court unless the legislation is withdrawn by the end of September.

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 its Brexit 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 ended with a historic peace deal in 1998.

"This protocol on the island of Ireland exists and one side cannot make it not exist. This is a unilateral, provocative act that is uniquely unprecedented," Ireland's Europe Minister Thomas Byrne said on BBC radio.

- 'Self-harm' -

Britain rejected the EU ultimatum, but said it remained committed to its obligations under the withdrawal treaty and to a protocol governing Northern Ireland's post-Brexit trade.

"It's not about if we implement the (EU) Withdrawal Agreement and the Northern Irish Protocol, it's how we implement it," business minister Nadim Zahawi said.

The new bill was an "insurance policy" if Britain and the EU cannot find agreement by the end of the transition period, he said, as "we can't allow any adverse impacts on the communities in Northern Ireland".

Despite the war of words this week, neither side wants to take the blame for abandoning negotiations that have been deadlocked over highly symbolic issues such as state aid for industry and fishing rights.

"We're in the final stretch of these negotiations, and for our part they will be carried through to the end," EU internal markets commissioner Thierry Breton told France's BFM television.

"We are not going to carry out any 'empty chair' diplomacy, that is quite clear," he said.

While the government tries to portray the bill as tidying up loose ends, several MPs and Conservative grandees are aghast at the prospect of Britain intentionally violating the treaty, and are warning of trouble when it comes before parliament next week.

Former Labour prime minister Gordon Brown said Johnson's stance was "a huge act of self-harm".

"We knew that there was a debate over fishing and over state aid, but then to explode the argument to breaking an international treaty is being condemned by so many people," he said.

Even ardent Brexiteers fretted about the bill's capacity to undermine Britain's reputation as a guarantor of the global rule of law, given its status as a permanent member of the UN Security Council.

And while boasting of its newly minted trade deal with Japan, the government was warned that the ructions over Northern Ireland could deprive it of another glittering prize -- a similar pact with the United States.

"How can they walk away from an international agreement? How do you trust that?" House of Representatives Speaker Nancy Pelosi said in Washington. AFP