More than 300 people are in intensive care with flu in England, Government figures have shown.
It is unclear how many of the 302 patients have swine flu but they are thought to be in the majority.
Health Secretary Andrew Lansley said the NHS can cope with an upsurge after the number of critical cases almost doubled in a week.
The country’s most senior doctor, Professor Dame Sally Davies, has urged people in at-risk groups – especially pregnant women – to come forward for the seasonal flu jab, which also protects against swine flu.
Prof Davies, the interim chief medical officer for England, said swine flu was the dominant strain in this year’s flu cases.
“We have not got a pandemic,” she added, saying a vaccine was available and some people are already immune.
Vaccination rates are slightly lower than last year.
In recent weeks, 14 people have died with confirmed swine flu and another three from flu type B. Last year, 474 people died from swine flu.
Prime Minister David Cameron told reporters at 10 Downing Street: “While obviously… the number of flu cases has doubled in the last week, there is still plenty of capacity in the NHS.”
But shadow health secretary John Healey said Mr Lansley should “get a grip” of the flu outbreak and criticised him for axing national advertising urging people to get the flu jab.
As of Monday, there were 24 children under five in critical care with confirmed or suspected flu, another 12 aged five to 15, and 243 in the 16 to 64 age group.