UPDATE 6-Biden warns UK on Brexit: No trade deal unless you respect N.Irish peace pact

* U.S. Congress official also cautions Johnson on a trade deal

* British prime minister worried about 'abusive' EU

* EU negotiator Barnier still hopes for a deal - sources

* Societe Generale sees 80% probability of a no deal (Adds BoE)

By Guy Faulconbridge and Elizabeth Piper

LONDON, Sept 17 (Reuters) - U.S. Democratic presidential candidate Joe Biden warned the United Kingdom that it must honour the Northern Irish peace deal as it extracts itself from the European Union or there would be no separate U.S. trade deal.

"We can't allow the Good Friday Agreement that brought peace to Northern Ireland to become a casualty of Brexit," he tweeted.

"Any trade deal between the U.S. and U.K. 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, but is proposing new legislation that would break parts of the Brexit divorce treaty relating to British-ruled Northern Ireland that seek to avoid a physical customs border.

The prime minister accuses the EU of trying to divide up the United Kingdom and putting a revolver on the table in talks to set rules for an estimated $1 trillion in annual trade.

He says the United Kingdom has to have the ability to break parts of the 2020 Brexit treaty if it is to uphold London's commitments under the 1998 pact which ended three decades of sectarian violence in Northern Ireland between pro-British Protestant unionists and Irish Catholic nationalists.

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 at the end of the year, and thus complicate the border between Northern Ireland and EU-member Ireland.

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.

'ABUSIVE' EU?

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."

Societe Generale analysts said on Thursday they now see an 80% chance that Britain and the EU will fail to strike a trade deal before the end of the year. That would create chaos for business and markets.

The Bank of England, which on Thursday kept interest rates unchanged, said market contacts had "reported renewed concerns over recent Brexit developments".

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 U.S. 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 U.S-British relations.

Engel also said Congress would not support a free trade agreement between the United States and the United Kingdom if Britain failed to uphold its commitments with Northern Ireland. (Reporting by Guy Faulconbridge, Elizbeth Piper, Sarah Young and Dhara Ranasinghe in London; and Gabriela Baczynska, Jan Strupczewski, John Chalmers in London; Editing by Sarah Young, Kate Holton, William Maclean, Andrew Cawthorne)