President-elect Joe Biden has said he does not want to see a guarded border between Ireland and the UK.
Mr Johnson’s government is seeking a trade deal with the European Union but says it is willing to leave without one.
That could complicate the situation at the sensitive Northern Irish border with Ireland – the UK’s only land border with the EU.
Mr Biden told journalists in Wilmington, Delaware, on Tuesday that the border must be open.
Mr Biden (pictured today) stressed the importance of protecting Northern Ireland’s peace deal in the Brexit process in a call with Boris Johnson this month after he won the US election
Mr Johnson (pictured yesterday) is seeking a trade deal with the European Union but says it is willing to leave without one
‘We do not want a guarded border,’ he said, answering a question from a reporter on what he would say to Brexit negotiators.
The 1998 Good Friday peace deal that effectively ended Northern Ireland’s 30 years of sectarian violence created institutions for cross-border cooperation on the island.
Mr Johnson put forward legislation in September 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.
Mr Biden, who has talked about the importance of his Irish heritage, warned months ago as the Democratic candidate for the presidency that the UK must honour the 1998 agreement as it withdraws from the bloc or there can be no separate US deal.
Mr Johnson has never met Mr Biden and commentators suggest the prime minister will have to work hard to foster the ‘special relationship’ between the historic allies.
Ireland’s prime minister said on Monday he hoped the outline of a Brexit free-trade deal would emerge by the end of this week, despite what the EU negotiator called ‘fundamental divergences’ at talks.
It comes after lorry drivers were left stuck in miles of M20 queues as French authorities tested Channel Tunnel border controls they say will be in place if there is no Brexit deal.
Getlink, which operates the Channel Tunnel, said the queues were ‘not representative’ of a no-deal Brexit, as half of the available lanes were blocked off.
Highways England also said it had closed the entry and exit sliproads at Junction 11 in Kent today due to the number of HGVs parked on the hard shoulder.
Lorries queueing at the M20 in Kent, waiting to access the Eurotunnel terminal in Folkestone. French authorities tested border controls they say will be in place if there is no Brexit deal
Freight lorries queueing along the M20 in Kent. Getlink said the queues were ‘not representative’ of a no-deal Brexit however, as half of the available lanes were blocked off
A spokesman for Getlink told MailOnline: ‘They [the tests] were carried out by French authorities to test the impact of EU and Schengen immigration controls in the event of a no deal.
‘The queues are not representative however, as they were only using half of the available lanes/booths available.’
The spokesman earlier said: ‘The backlog (of traffic) built up this morning during tests conducted by the authorities in preparation for Brexit.’
They added this afternoon that traffic was now flowing ‘smoothly’ through the terminal and services were operating as normal.
It is understood that further intermittent border tests will be carried out before the Brexit transition period comes to an end on December 31.
Photographs showed dozens of freight lorries queued along the M20.
Freight lorries queuing along the M20, with two ambulances also seen in one of the lanes. Getlink said this afternoon that traffic was now flowing ‘smoothly’ through the terminal
The delays were caused by French authorities testing the impact of EU and Schengen immigration controls at the Eurotunnel terminal in Folkstone in the event of no deal
Freight lorries queuing along the M20 in Kent, while half of the available lanes were blocked off. It is understood that further intermittent border tests will be carried out before the Brexit transition period comes to an end on December 31
Highways England tweeted: ‘M20 coastbound at J11 Westenhanger – the entry slip is now closed due to heavy congestion in the area due to delays at Eurotunnel.
‘Due to the heavy congestion with HGV’s at this location, for safety reasons we have also closed the exit slip at J11 Westenhanger due to the amount of lorry’s parking up along the hard shoulder all the way back to exit slip.’
A spokesperson for The Road Haulage Association told MailOnline: ‘There is much speculation as to how the motorway network leading to and from the ports will work with or without a deal.
‘The subsequent delays or congestion are unavoidable, but necessary at this time as testing is in progress.’
The Channel Tunnel connects Folkestone to Coquelles near Calais in France.