The people have been quarantined, according to local media, with officials working to confirm the routes of infection and check whether there are close contacts who could be at risk of infection.
Professor Neil Ferguson, a member of the government’s Scientific Advisory Group for Emergencies, said this week that the variant is “almost certainly” in a “great majority” of European countries.
Countries have begun closing their borders to UK travellers in an attempt to curb the spread of B.1.1.7.
Earlier this week, Japan banned the entry of foreign nationals who have been in the UK in the past 14 days.
The country’s immigration authority has also suspended the exemption of a 14-day self-quarantine measure for short-term business travellers who have returned to Japan from Britain.
And beginning Sunday, the government will require all Japanese citizens to submit upon entry a negative result from a virus test taken within 72 hours before departure.
Prime minister Yoshihide Suga urged the nation on Friday to spend a quiet New Year period without the usual social gatherings to prevent the spread of Covid-19, which has been breaking infection records almost on a daily basis.
Mr Suga also announced a package of £1.9bn for hospitals treating patients which have come under strain due to the rapid rise in cases across the northern island of Hokkaido, as well as large cities like Tokyo and Osaka.
"I want you to spend a quiet New Year," the PM told a news conference in Tokyo, alongside the government’s leading coronavirus expert Shigeru Omi.
"The infections aren't coming down and if we keep going like this, we won't be able to avoid further spread of the virus."
Japan does not celebrate Christmas, but the New Year period is an extended national holiday, with many people usually travelling back to their hometowns and spending time with family and friends.
Mr Omi warned that it was critical that "all citizens move in the same direction", to get a grip on the health crisis.
"If we don't bring infections down now, once they surge again after the New Year period it won't be easy to change the downward trend," he said. "It would take time, and would probably be impossible to control over a period of weeks," he said.
Mr Omi said shared meals were a major cause of infections and called on people to refrain from holding large gatherings and to limit meals to four people, or fewer.
While Japan has avoided the huge infection numbers seen in other parts of the world, the number of new daily cases surpassed 3,000 for the first time this month. Tokyo reported 884 infections on Friday.
Underscoring the strain on the hospitals, five national groups of doctors and other medical workers made an emergency request to Mr Suga on Friday, asking for strong anti-pandemic measures and support for the medical sector.
With hospitals equipped for treating Covid-19 filling up, other hospitals are being forced to accept patients with the disease, according to Tsuyoshi Masuda, president of the Japan Federation of Democratic Medical Institutions.
"These small and medium-sized hospitals, which have been supporting medical services in their respective regions, are facing a crisis that is threatening their survival," Mr Masuda told reporters at a separate news conference on Friday.
He also warned that the risk of in-hospital infections was high at institutions not specialised in dealing with infectious diseases.
Japan, with a population of 126 million, has struck deals to buy 290 million vaccine doses from Pfizer, AstraZeneca and Moderna, or enough for 145 million people.
A health ministry panel said that people aged 65 or older should get priority for vaccination against Covid-19, as well as frontline healthcare workers and people with underlying medical conditions.
It specified chronic heart disease, chronic respiratory disease and chronic kidney disease, among others, as underlying conditions that should determine priority.
The panel’s recommendations would mean 36 million elderly people and 8.2 million people with medical conditions would be the first to receive shots.
Additional reporting by Reuters
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