The estimated number of new cases of swine flu has almost doubled this week, raising fears that increasing hospital admissions will put pressure on NHS beds.
The number of deaths of people in the UK suffering from swine flu has risen to 128, with 93 deaths in England, 21 in Scotland, eight in Northern Ireland and six in Wales.
But Chief Medical Officer Sir Liam Donaldson said he was most worried by the number of hospital cases, which have risen in England to 506, with 99 in critical care.
The number in critical care is the highest since the pandemic began.
Sir Liam said: "We don't understand why this has happened. It looks as if the virus is having a different impact in the flu season than it had in the summer."
He said he was concerned by the high proportion of deaths in younger age groups, describing it as a "very unusual pattern for flu".
More than half (54%) of deaths have been in the under-45 age group while 77% have been in the under-65 group. With seasonal flu the majority of deaths would be expected in the over-65s. One in three deaths related to people who had little or no underlying health problems.
Sir Liam said: "It's highly unusual to have so many younger people dying. We should not pass this off as an acceptable number."
The estimated number of new cases this week was 53,000, "almost doubling" from last week when new cases were estimated at 27,000. The total number of cases is estimated at 435,000.
The number of cases could peak at 1.5 million people ill in a single week, putting prolonged pressure on the NHS over the winter. There could be 35,000 more hospital admissions, with 15% of patients needing critical care. If such a scenario unfolds, there could be a total of 1,000 deaths and 5% of working people off sick.
