The number of illness cases peaked at 500,000 during the second week of January, The Daily Telegraph reported Saturday.

This year, the norovirus season began a month earlier than in previous years, officials said.

The Health Protection Agency confirmed 3,534 cases since July and said for every case reported at least 1,500 go unreported.

Experts said the illness is not usually dangerous. Doctors advise people experiencing vomiting and diarrhea to not leave home and to drink a lot of liquids, the newspaper said.

In January, a quarter of British hospitals had to close down certain wards to quarantine people with the bug.

"The evidence is that norovirus is increasing each year," Royal College of General Practitioners Steve Field said.