The doubling of swine flu cases has prompted fears over the increasing pressure they are putting on hospital critical care facilities.
Deaths have risen to 128 and admissions in England have increased to 506, with 99 of them critical - the most since the pandemic began.
Chief Medical Officer Sir Liam Donaldson is most concerned by the high proportion of deaths in younger age groups, describing it as a "very unusual pattern for flu".
More than half have been in the under-45 age group and 77% in the under-65 group. With seasonal flu, the majority would be expected in the over-65s.
One-third of deaths are of people who had little or no underlying health problems. Says Sir Liam: "We don't understand why this has happened".
"It's highly unusual to have so many younger people dying. We should not pass this off as an acceptable number. It looks as if the virus is having a different impact in the flu season than it had in the summer," he said.
The estimated number of new cases this week was 53,000, almost double last week's 27,000. The total number is estimated at 435,000.