Swine flu has struck 40 children at Birmingham's Welford Primary School, prompting the Health Protection Agency (HPA) to stress that none are seriously ill.
All are being treated with anti-viral drugs, and the HPA says that the school is being "deep cleaned" prior to reopening next week after the holidays.
Said a spokeswoman: "There are still a number of laboratory results outstanding and the number of confirmed cases at the school is expected to rise."
She said that all the 50 confirmed cases in Birmingham are being treated at home with anti-viral drugs and are responding well to treatment.
The scale of the outbreak emerged as the Department of Health said that the total number of cases confirmed throughout the UK has now risen by 47 to 184.
According to the NHS, most UK cases are either linked to previously confirmed cases or are returning travellers. So far, all UK cases have been mild.
As the global number of confirmed swine flu cases passes 12,000, the alert level from the World Health Organization (WHO) remains at 5, meaning that there is human-to-human spread of the virus in at least two countries in the same region.