US Democratic presidential candidate Joe Biden warned the United Kingdom that it must honour Northern Ireland's 1998 peace agreement as it withdraws from the European Union or there would be no separate US trade deal.
Prime Minister Boris Johnson is proposing new legislation that would break the Northern Ireland protocol of the Brexit divorce treaty that seeks to avoid a physical customs border between British-ruled Northern Ireland and EU-member Ireland.
"We can't allow the Good Friday Agreement that brought peace to Northern Ireland to become a casualty of Brexit," Biden said in a tweet on Wednesday.
"Any trade deal between the US and UK must be contingent upon respect for the Agreement and preventing the return of a hard border. Period."
Johnson insists he is defending not threatening the Good Friday pact, which ended three decades of sectarian violence in Northern Ireland between pro-British Protestant unionists and Irish Catholic nationalists.
He accuses the EU of trying to divide the United Kingdom and of putting a revolver on the table in talks to set rules for an estimated $1 trillion in annual trade after Britain's post-Brexit transition period expires at year-end.
"The PM has been clear throughout that we are taking these steps precisely to make sure that the Belfast Agreement is upheld in all circumstances and any harmful defaults do not inadvertently come into play," Johnson's spokesman said.
The prime minister says the United Kingdom must have the ability to break parts of its Withdrawal Agreement with the EU if London is to uphold commitments under the 1998 peace deal.
The EU says any breach of the Brexit treaty could sink trade talks, propel the United Kingdom towards a messy exit when it fully departs on Dec. 31, and thus complicate the Northern Irish border, the country's only land frontier with the EU.
The EU's Brexit negotiator Michel Barnier told the bloc's 27 national envoys on Wednesday that he was still optimistic, three diplomatic sources told Reuters.
"Barnier still believes a deal is possible though the next days are key," said one of the EU sources.
Johnson told The Sun newspaper that the EU must not be allowed to abuse Britain and risk four decades of partnership.
He said the United Kingdom must "ring-fence" the Brexit deal "to put in watertight bulkheads that will stop friends and partners making abusive or extreme interpretations of the provisions". He used the example of a potential EU demand for tariffs on food going to Northern Ireland from Britain.
Societe Generale analysts said on Thursday they now see an 80 percent chance that Britain and the EU will fail to strike a trade deal before the end of the year.
The Bank of England, which on Thursday kept interest rates unchanged, said market contacts had "reported renewed concerns over recent Brexit developments".
France warned against collapsing Brexit trade talks -- a step it said could suit some in London.
"We shouldn't fall into the trap," said Clement Beaune, France's European affairs minister.
'DON'T LECTURE UK'
The intervention by Biden, who nationwide polls show leading the race for Nov. 3's US election, prompted a sharp rebuke from an ex-leader of Johnson's Conservative Party, Iain Duncan Smith, who advised him to focus on "riots" rather than Brexit.
"We don't need lectures on the Northern Ireland peace deal from Mr Biden," Duncan Smith told The Times. "If I were him I would worry more about the need for a peace deal in the USA to stop the killing and rioting before lecturing other sovereign nations."
Biden, who has talked about the importance of his Irish heritage, retweeted a letter from Eliot Engel, chair of the Foreign Affairs Committee of the US House of Representatives, to Johnson exhorting him to honour the Good Friday deal.
Engel urged Johnson to "abandon any and all legally questionable and unfair efforts to flout the Northern Ireland protocol of the Withdrawal Agreement" so as to preserve peace in the province and good US-British relations.