Germany said all flights coming from Britain, except cargo flights, were no longer allowed to land starting midnight Sunday. It didn't immediately say how long the flight ban would last.
Belgian Prime Minister Alexander De Croo said he was issuing a flight ban for 24 hours starting at midnight "out of precaution".
"There are a great many questions about this new mutation," he said, adding he hoped to have more clarity by Tuesday.
A senior Canadian Government official said on Sunday evening that Canada would also ban flights from Britain. The official, who spoke on condition of anonymity as they were not authorised to speak publicly ahead of a formal announcement, said the ban would take effect Monday.
The British Government said Johnson would preside at a meeting of the Government's crisis committee, COBRA, on Monday in the wake of the other nations' measures.
They come at a time of huge economic uncertainty for the UK, less than two weeks before it leaves the EU's economic system on December 31, and with talks on a new post-Brexit trade relationship still deadlocked.
Johnson said Saturday that the fast-moving new variant of the virus was 70 per cent more transmissible than existing strains. But he stressed "there's no evidence to suggest it is more lethal or causes more severe illness," or that vaccines will be less effective against it.
The UK recorded 35,928 further confirmed cases on Sunday, around double the number from a week ago.
Germany called a special crisis meeting on Monday to co-ordinate the response to the virus news among the EU bloc's 27 member states.
The Netherlands banned flights from the UK for at least the rest of the year. Ireland issued a 48-hour flight ban. Italy said it would block flights from the UK until January 6, and an order signed Sunday prohibits entry into Italy by anyone who has been in the UK in the last 14 days.
The Czech Republic imposed stricter quarantine measures from people arriving from Britain. Beyond Europe, Israel also said it was banning flights from Britain, Denmark and South Africa because those were the countries where the mutation is found.
Viruses mutate regularly, and scientists have found thousands of different mutations among samples of the virus causing Covid-19. Many of these changes have no effect on how easily the virus spreads or how severe symptoms are.
Britain has seen over 67,000 deaths in the pandemic, the second-highest confirmed toll in Europe after Italy.
Europe as a whole has recorded nearly 499,000 virus deaths, according to a tally by Johns Hopkins University that experts believe is an undercount, due to limited testing and missed cases.
