A sharp increase in the number of patients with swine flu admitted to critical care, many of them young, was highlighted by government experts yesterday as they warned the NHS faced a tough winter ahead. The number of deaths in the UK has risen to 128, more than half of them of people aged under 45. In England the number of patients in critical care has risen to 99, the highest since the pandemic began.

But the warning came with a silver lining as revised "worst-case" assumptions for the pandemic show that up to 12 per cent of the population are expected to catch the virus, down from 30 per cent, and 1,000 people in the UK may die, down from 19,000. Children are expected to be disproportionately affected with up to one in three catching the disease.

A death toll of 1,000 would be substantially below that from ordinary seasonal flu, which has ranged from 4,000 to 8,000 in recent years, and rose to 35,000 in the last major epidemic in 1989. However, the progress of swine flu was giving concern because of the very severe illness it caused in some people, especially the young.

Sir Liam Donaldson, the chief medical officer, giving his weekly briefing on the pandemic, said the high number of people in critical care was worrying. "We don't understand why this has happened," he said. "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. "It's highly unusual to have so many younger people dying. We should not pass this off as an acceptable number," he said.

The estimated number of new cases in the last week was 53,000, almost twice the previous week when new cases were estimated at 27,000. Sir Liam said: "We are starting to worry about the sustained pressure over the winter that the NHS might face. It's a very long period for the NHS to sustain its response. There is concern about the continuing pressure that the NHS is likely to be under this winter from swine flu over a sustained period of time but also from other winter illnesses including seasonal flu."

Ian Dalton, national director of NHS flu resilience, said the NHS had put an "unprecedented amount" of work into preparing for the virus and to be able to increase the number of critical care beds where necessary.