U.S. Democratic presidential candidate Joe Biden has warned the United Kingdom that it must honour the Northern Irish peace deal as it extracts itself from the European Union or there will be no U.S. trade deal.

“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.

“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.”

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 United Kingdom.

Biden retweeted a letter from Eliot Engel, chair of the Foreign Affairs Committee of the U.S. House of Representatives, that called on Johnson to honour the 1998 Good Friday peace deal.

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

He called on 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.”

Engel 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.

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

Johnson says the United Kingdom has to break parts of the 2020 Brexit treaty to uphold its commitments under the 1998 peace deal.