US Democratic presidential candidate Joe Biden warned the United Kingdom that it must honour the Northern Irish peace deal as it extracts itself from the European Union or there would be no separate US trade deal.
"We can't allow the Good Friday Agreement that brought peace to Northern Ireland to become a casualty of Brexit," he tweeted.
"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."
Johnson insists he is defending not threatening the Good Friday pact, but is proposing new legislation that would break parts of the Brexit divorce treaty relating to British-ruled Northern Ireland that seek to avoid a physical customs border.
The prime minister accuses the EU of trying to divide up the United Kingdom. He says the United Kingdom has to have the ability to break parts of the 2020 Brexit treaty if it is to uphold London's commitments under the 1998 pact 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, propel the United Kingdom towards a messy exit when it fully departs at the end of the year, and thus complicate the border between Northern Ireland and EU-member Ireland.
On Wednesday, British government reached a deal 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 break international law.