The surprise intervention by Mr Biden will pile pressure on Mr Johnson following his decision to press ahead with the Internal Market Bill, which re-opens parts of the Brexit withdrawal agreement and threatens to break international law.
The Prime Minister insists the legislation acts as a “safety net” to the peace process and ensures Brussels could not impose tariffs on goods moving to Northern Ireland from the rest of the UK in the event of no deal.
Prior to the comments by Mr Biden, four US congressmen, led by chairman of the House Foreign Affairs Committee Eliot Engel, wrote to the Prime Minister and said Washington will not support any free trade pact with the UK if Britain fails to uphold its commitments to Northern Ireland as part of Brexit.
In the letter also signed by Democrats Richard Neal and William Keating and Republican Peter King, they said: “Many in the US and in Congress consider the issues of the Good Friday Agreement and a potential US-UK Free Trade Agreement inextricably linked.”
Read More: President hopeful Joe Biden wades into Brexit row with trade deal warning to