New cases of swine flu rose by 50% in England and doubled in Scotland last week, indicating the start of a second wave of infections, say British health officials.
There were about 14,000 new cases of the H1N1 influenza virus in England, the Health Protection Agency (HPA) said.
In Scotland, the number of people contracting the H1N1 rose to 13,800, double last week's infection rate.
England's Chief Medical Officer Liam Donaldson told BBC television that this was the start of the second peak of the virus, which he said was not changing in character.
"Most people get a mild illness from which they make a good recovery, but a small proportion get very serious complications...and some are previously healthy people," he said.
"So we do have to be cautious about passing this off as a mild illness. It's not a mild illness for everybody."
The HPA said the number of deaths related to swine flu in England stood at 72. In Scotland, the government said 10 people who contracted the virus had died.
Mr Donaldson said the Department of Health had been given the green light from European authorities to start using one of the vaccines and expected to start administering it sometime in the second half of October.
He also said Northern Ireland had seen a much steeper rise than England.