Government figures show swine flu cases in England increased from around 3,000 to 5,000 last week.
The data suggested an upturn is on its way according to chief medical officer for England, Sir Liam Donaldson.
A second wave of swine flu has been predicted to hit the UK in the winter months according to experts, following a lull over the summer.
Cases of the illness also increased in Scotland last week.
Sir Donaldson said: "We don't know whether it will be sustained but I think it's one of the few straws in the wind that we might be seeing an upturn."
In England, 143 people are being treated in hospital with swine flu, of whom 23 are in intensive care.
There was one death in England last week linked to the virus, bringing the total number of deaths to 67.
Copyright © Press Association 2009
NHS Swine Flu
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