New figures showing new cases of swine flu almost doubled this week have sparked concerns over the strain it could place on the NHS.
The UK swine deaths tally has increased to 128, with 93 deaths in England, 21 in Scotland, eight in Northern Ireland and six in Wales.
But it is the amount of hospital cases which is most worrying for Chief Medical Officer Sir Liam Donaldson – which have shot up in England to 506, with 99 in critical care, the highest amount 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.
Sir Liam said: "It's highly unusual to have so many younger people dying. We should not pass this off as an acceptable number."