Britons on a charity trip to Kenya tested positive for swine flu, the Foreign Office said.
A man tested positive in the city of Kisumu on Sunday and further members of his group - thought to be students from Nottingham University on a medical field trip to the western African nation - have since also contracted the H1N1 virus.
The Foreign and Commonwealth Office said the group was being kept in isolation.
The numbers in the group with swine flu could not be confirmed as they were being re-tested, but they are receiving treatment.
The flu symptoms so far were said to be mild and have not required hospital treatment.
The British High Commission was in contact with the student group and authorities in Kenya, which is experiencing what are now thought to be the country's first cases of the swine flu.
The group was visiting projects that a wider collective of students in the UK has been fundraising for and supporting, the Foreign Office said.
The Kenyan Ministry for Public Health and Sanitation said 34 British citizens had travelled to Kisumu on June 21 to undertake field studies.
Two days later, a 20-year-old male student at Nottingham University developed a headache and joint pains and handed himself over to Kenyan health authorities.
The man's girlfriend had contacted him eight days ago from Nottingham to tell him she had tested positive for swine flu.