The number of swine flu cases in the UK is probably double the official figure, a leading bacteriologist has said.
The official total stands at well above 500 after more cases were confirmed on Saturday, but Prof Hugh Pennington told BBC 5 Live the real figure is higher.
"I think it would be very reasonable to say we've got at least twice as many cases as we know about," he said.
Professor Pennington, a bacteriologist at Aberdeen University, said because swine flu was similar to seasonal flu, there would be many more undeclared cases.
"All the experts think that the numbers we've got are a significant underestimate," he said.
"In the United States, they think for every case they know about, every confirmed case, there are 20 other cases out there in the community.
"I don't think we're anywhere near that, but I think it would be very reasonable to say we've got at least twice as many cases as we know about, and it may be more than that."