LONDON (REUTERS) - United States Democratic presidential candidate Joe Biden warned the United Kingdom that it must honour the Northern Ireland peace deal as it extracts itself from the European Union or there will be no US trade deal.

"We can't allow the Good Friday Agreement that brought peace to Northern Ireland to become a casualty of Brexit," Mr Biden said in a tweet.

"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."

Prime Minister Boris Johnson unveiled legislation last week that would break parts of the Brexit divorce treaty relating to Northern Ireland, blaming the EU for putting a revolver on the table in trade talks and trying to divide the UK.

He says the UK has to have the ability to break parts of the 2020 Brexit treaty he signed to uphold London's commitments under the 1998 peace deal 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 and thus complicate the border between the UK's Northern Ireland and EU-member Ireland.

Asked about Mr Biden's criticism, British junior health minister Edward Argar said that he did not believe the Northern Irish peace deal was at risk.

Mr Johnson told The Sun that the EU was being "abusive" to Britain and risking four decades of partnership.

He said the UK 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".

Mr Biden, who has talked about the importance of his Irish heritage, retweeted a letter from Mr Eliot Engel, chair of the Foreign Affairs Committee of the US House of Representatives, that called on Mr Johnson to honour the 1998 Good Friday peace deal.

Mr Engel urged Mr Johnson to "abandon any and all legally questionable and unfair efforts to flout the Northern Ireland protocol of the Withdrawal Agreement".

He called on Mr Johnson to "ensure that Brexit negotiations do not undermine the decades of progress to bring peace to Northern Ireland and future options for the bilateral relationship between our two countries".

Mr Engel said Congress would not support a free trade agreement between the US and the UK if Britain failed to uphold its commitments with Northern Ireland.

The letter was signed by representatives Richard Neal, William Keating and Peter King.

Mr Johnson is pushing ahead with his plan.

His government reached a deal on Wednesday to avert a rebellion in his own party, giving Parliament a say over the use of post-Brexit powers within its proposed Internal Market Bill that breaks international law.