The number of swine flu cases in England has jumped in the last week, from an estimated 3,000 to 5,000, Government figures show.
Chief medical officer for England Sir Liam Donaldson said the data suggested an upturn was on the way.
Experts have been predicting a second wave of swine flu will hit the UK in the winter months following a lull over the summer break.
Scotland has also seen a rise in the number of cases in the last week.
Sir Liam said: “‘There has been a slight upturn this week. 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.’
There are now 143 people in hospital in England, of whom 23 are in intensive care.
The total number of deaths linked to the virus has risen by one in the last week to 67 in England.