The Tokyo Metropolitan Government says three people who returned to Japan from Britain and one of their family members have tested positive for the coronavirus.
Samples from the individuals will undergo genetic screening at the National Institute of Infectious Diseases to see whether they are infected with a new variant spreading in the UK.
Officials in Tokyo say the infections of two women in their 20s and a man in his 30s were confirmed after they returned from Britain between December 14 and 17.
They say the three initially tested negative upon entry to Japan, but they later developed symptoms and were found by medical institutions in Tokyo to be infected.
Officials say a family member who was in close contact with the man in his 30s has also tested positive.
Three of the four have been hospitalized, and the remaining person is due to be admitted on Friday. None are said to be in serious condition.
The variant of the coronavirus confirmed in Britain is said to be more transmissible. Tokyo Governor Koike Yuriko told reporters that she will work with the central government to enforce border controls.