U.S. Democratic presidential candidate Joe Biden issued a warning for the United Kingdom that any future trade deal between the two countries was dependent on upholding of the agreement that brought peace to Northern Ireland. In a tweet posted on Wednesday (September 16) Biden said “We can't allow the Good Friday Agreement that brought peace to Northern Ireland to become a casualty of Brexit.” He added that any agreed trade deal must prevent the return of a hard border on the island of Ireland. Biden’s comments come on the back of UK prime minister Boris Johnson unveiling legislation that would break parts of the existing Brexit divorce treaty relating to Northern Ireland. His tweet included a screenshot of a letter to Johnson written by the chair of the Foreign Affairs Committee of the U.S. House of Representatives, which 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 European Union says any breach of the Brexit treaty could damage trade talks and complicate the border between the United Kingdom's Northern Ireland and EU-member Ireland.