By Nick Triggle
Health reporter, BBC News
Sir Liam Donaldson: "We might be beginning to see the beginning of this second peak"
Swine flu looks to be on its way back, according to Sir Liam Donaldson, England's chief medical officer.
Figures showed that there were an estimated 5,000 new cases in England last week - up from 3,000.
It is the first increase since the end of July, but the levels are still below what would be expected in a bad winter.
In Scotland, the numbers have nearly doubled to more than 6,000, although in Wales and Northern Ireland the figures are much lower.
Most of the rise in England is accounted for by school children with six schools reporting suspected outbreaks.
A second wave of swine flu had been predicted to hit the UK in the winter months following the summer lull.
Sir Liam said this now appeared to be happening.
He added: "It does begin to suggest swine flu is coming back.
"We would naturally have hoped for a bit more breathing space before it started again."
He said the rise showed why it was important to get the vaccination programme under way quickly.
A deal has now been agreed with GPs, meaning the UK will be ready to roll out the programme as soon as regulators license the jab.
This is likely to happen in early October.
The number of deaths in the UK has also risen to 79 - four more than last week - with more than 150 people in hospital because of the virus.
Meanwhile, the Health Protection Agency has reported one confirmed case and one possible case of resistance to the flu drug Tamiflu.
There has been fewer than 30 cases like this worldwide with experts monitoring them for signs that the virus is mutating.
However, both of the UK cases showed no signs of passing the virus on so officials said they were not overly concerned.