Tens of thousands of Londoners will be hit by a second wave of swine flu within three weeks, it was claimed today.

Experts warned that the pandemic is likely to be "at its peak" in the capital between the beginning of October and the end of November.

Last week the Chief Medical Officer, Sir Liam Donaldson, predicted that the next phase of the virus could kill 19,000. At the height of the first wave, more than 100,000 Londoners a week were reporting flu-like symptoms. So far 61 people have died with swine flu in Britain - 29 of those in the capital.

John Edmunds, professor of infectious disease modelling at the London School of Hygiene and Tropical Medicine who is researching swine flu cases, said he expected the second wave of the virus to hit before Christmas.

Over the next two months, Londoners will be tracked on www.flusurvey.org.uk to try to contain the virus and obtain more accurate figures. "About 15 per cent of the population will be ill," Professor Edmunds said.

The number of people infected with swine flu in June and July is likely to be higher than the Health Protection Agency statistics say because of people failing to report symptoms, he said.

Research suggests that school closures could lead to greater rates of infection among older people. Professor Edmunds said: "We can be fairly confident that closing schools will reduce the amount of contact with other children. But how much it changes their patterns of contact with others, we don't know. You can guess it might increase their rate of contact with adults."

The school of tropical medicine, University College London and King's College School of Medicine are part of a £2.25million project to work out how the second wave will spread. Professor Steve Field, chairman of the Royal College of GPs, said: "Given that the weather is cooler and schools are back, you would expect there will be an increase. The information on swine flu became less accurate when we started recording it as influenza-like illness. On one hand the [National Pandemic Flu Service] helpline has had a positive effect, but it messes up the statistics."

Official figures show that the number of suspected swine flu cases has increased by a third - from just over 4,000 last week to 5,200. The rise was mainly in school-age children, who have returned for autumn term.

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