PM Johnson congratulates Joe Biden, a win that could pose him problems

Johnson, dubbed "Britain Trump" by the incumbent Donald Trump, has never met Biden and commentators have suggested he will have to work hard to foster the so-called "special relationship" between the close allies.

"Congratulations to Joe Biden on his election as President of the United States and to Kamala Harris on her historic achievement," Johnson said in a statement.

"The U.S. is our most important ally and I look forward to working closely together on our shared priorities, from climate change to trade and security."

However, Biden's election could pose an immediate difficulty for Johnson, whose government is seeking a trade deal with the European Union.

Biden, who has talked about the importance of his Irish heritage, has warned that the United Kingdom must honour Northern Ireland’s 1998 peace agreement as it withdraws from the bloc or there can be no separate U.S. trade deal.

That message came after Johnson put forward legislation that would break the Northern Ireland protocol of the Brexit divorce treaty that seeks to avoid a physical customs border between the British province and EU-member Ireland.

He argues the law is needed to uphold the terms of the Good Friday agreement.

In addition to receiving lavish praise from Trump, Johnson was accused of insulting Barack Obama, under whom Biden was vice-president, in a 2016 newspaper article.

He suggested a bust of British wartime leader Winston Churchill had been moved from the Oval Office because of the "part-Kenyan president’s ancestral dislike of the British Empire".

However, commentators also suggest that there would be much in policy terms that the New-York born Johnson and Biden would agree on, from tackling climate change and the Iran nuclear deal, to support for NATO, and security and intelligence.

British Foreign Secretary Dominic Raab, who said it was clear Biden had won despite some processes "still playing out", also emphasized where the allies could work together.