Chief medical officer Sir Liam Donaldson has spoken of his concern over the doubling of swine flu cases.

New estimates reveal the number of new cases of the swine flu virus has almost doubled, and as the NHS prepares for the winter flu season fears are increasing over the pressure on NHS beds.

It’s highly unusual to have so many younger people dying. We should not pass this off as an acceptable number

Sir Liam Donaldson said the rise in the number of hospital cases to 506 was most worrying, with the number in critical care reaching 99, the highest since the pandemic started.

The number of deaths of people suffering from the illness in the UK has risen to 128, including 93 deaths in England, 21 in Scotland, eight in Northern Ireland and six in Wales.

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 per cent) of deaths have been in the under-45 age group, while 77 per cent are in the under-65 group.

One in three deaths related to people who had few or no underlying health problems. With seasonal flu, the majority of deaths would be expected in the over-65s.

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.