The Department of Health has confirmed a further 15 cases of swine flu in England, bringing the total number of cases in the UK to 557.
However, a leading scientist has warned that the actual number could be twice as many as the current total.
Professor Hugh Pennington, a bacteriologist at Aberdeen University, said: "I think it would be very reasonable to say we have got at least twice as many cases as we know about.
"All the experts think that the numbers we have got are a significant underestimate."
He added that in the US there were probably 20 cases of swine flu for each confirmed case, but added: "I don't think we are anywhere near that, but I think it would be very reasonable to say we have got at least twice as many cases as we know about, and it may be more than that."
The warning comes as more people were diagnosed with the virus in England and 13 more cases were confirmed in Scotland, as well as one more in Northern Ireland.
Of the newly confirmed cases in England, seven are in London, three in the north west and two in the south east. The east of England, East Midlands and West Midlands each have one new case.