France has joined Ireland and a growing list of European countries to impose travel restrictions on the UK - forcing the prime minister to call an emergency COBRA meeting.
A Number 10 spokesperson said: "The prime minister will chair a COBRA meeting tomorrow to discuss the situation regarding international travel, in particular the steady flow of freight into and out of the UK."
It comes after France suspended "all flows of people from the United Kingdom for 48 hours, and for all means of transport".
A government spokesman said the ban also includes all incoming accompanied freight by road, air, sea or rail.
The Port of Dover has since closed to "all accompanied traffic leaving the UK until further notice due to border restrictions in France".
Watch: Pound weakens as markets react to Europe's UK travel bans
And Eurostar tweeted: "Following the announcement by the French government that the border with the UK will close at midnight tonight, we will only be able to run trains from Paris to London for the 21st-22nd December."
Transport Secretary Grant Shapps tweeted: "We're asking the public & particularly hauliers not to travel to Kent ports or other routes to France.
"We expect significant disruption in the area. My department is urgently working with Highways England and Kent Council on contingency measures to minimise traffic disruption in the area."
Ireland's restrictions on flights and ferries will last for an initial 48 hours before being reviewed during a cabinet meeting on Tuesday.
Several mainland European countries are also imposing bans - including Germany.
A government spokesman said it was in contact with its European partners about the travel restrictions, but it was not immediately clear when or for how long they would last.
The Netherlands has banned flights for at least the rest of the year and will assess "with other European Union nations the possibilities to contain the import of the virus from the United Kingdom".
Italy's foreign minister Luigi Di Maio announced curbs were being imposed.
Belgium's prime minister issued a ban for at least 24 hours while the situation was assessed.
Bulgaria will suspend flights to and from the UK at midnight until 31 January.
Turkey, Austria and the Czech Republic are also imposing new measures against UK flights, with Prague announcing that people arriving in the country having spent at least 24 hours in UK territory will now need to self-isolate.
Boris Johnson said the fast-moving new variant of the virus, which he added is thought to be 70% more transmissible than existing strains, appears to be driving a rapid spread of new infections.
The prime minister put London and much of the South East, where the new strain is most prevalent, into a Tier 4 lockdown over the Christmas period in an attempt to get the disease back under control.
Watch: How worried should we be about the threat of the new strain?
Viruses mutate regularly, and scientists have found thousands of different of mutations among samples of the virus causing COVID-19.
But many of these changes have no effect on how easily the virus spreads or how severe symptoms are.
Britain has alerted the World Health Organisation over the new variant identified this week, saying that it accounted for some 60% of London's cases.
Health Secretary Matt Hancock has warned the country is facing an "enormous challenge", adding that the mutation was "out of control" and could see areas stuck in Tier 4 until the UK's most vulnerable have been vaccinated.
Dr Susan Hopkins, of Public Health England, said the new variant had spread to every region of England - as well as parts of Scotland and Wales, but in smaller amounts.
She also said there was no evidence it was causing a disproportionate number of hospital admissions.
Meanwhile, the European Medicines Agency (EMA) is meeting on Monday to approve the first COVID-19 vaccine for the European Union's 27 nations, bringing vaccinations closer for millions of EU citizens.
The vaccine made by German pharmaceutical company BioNTech and American pharmaceutical company Pfizer is already in use in the US, Britain, Canada and other countries.