The former vice-president and Democrat presidential nominee has told the Prime Minister any post-Brexit trade deal between the two nations is “contingent upon respect for the Agreement”.
Mr Biden wrote on Twitter: “We can’t allow the Good Friday Agreement that brought peace to Northern Ireland to become a casualty of Brexit.
“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.”
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.
This is a developing news story, more to follow.