A pig farm in Canada has seen the first documented instance of swine flu being transmitted from humans back into animals, raising concerns that the H1N1 virus at the centre of the outbreak may mutate into a more virulent form as it passes between pigs and humans.

A Canadian farm worker transmitted the H1N1 virus to the swine herd in Alberta after a trip to Mexico. About 200 pigs from a herd of more than 2,000 developed symptoms, but none have died, health officials said yesterday.

The World Health Organisation was informed of the human-to-pig transmission on Saturday and its officials have since called for increased surveillance of pig farms, especially in countries with H1N1 infection in humans. "If this happened once, it could happen again," a WHO spokesman said.

Scientists are concerned that the recycling of the H1N1 virus by repeated transmission between humans and pigs may increase the chances of it mutating into a more virulent form that could lead to a higher mortality rate in humans during later waves of infection.

The WHO said that the virus isolated from the Alberta pig farm seemed to be the same as the one in humans. However, there is a risk that the current wave of "mild" flu may die down in the northern hemisphere this summer and return in the autumn in a more aggressive form.

As of yesterday, 787 confirmed cases of H1N1 flu had been recorded in 17 countries, though mostly in Mexico and the US, a WHO spokesman said.

In Britain, the Department of Health announced three new cases: a man in Ayrshire and two school-aged children in London - bringing the total in the capital to five.

"A 14-year-old child from Barnet has been found to have the virus following contact with a visitor to Mexico. An 11-year-old child in Wandsworth has been tested and found to have the virus following a visit to the United States," a health spokesman said.

"Both cases are being treated at home with antiviral drugs and are responding to treatment. The Health Protection Agency is carrying out further tests on people who have had close contacts with these patients," he said.

However, the official figures may not be up to date. Kate Corbett, 29, from west London, was told yesterday by the Health Protection Agency that she has confirmed swine flu after a visit to Mexico, but her case had not yet been registered by the Department of Health.

The WHO said that it did not plan to increase the pandemic level from phase 5 to phase 6, the highest alert level, indicating that a full-blown pandemic is under way. For phase 6, sustained, community-wide transmission within at least two WHO regions has to be confirmed.

Sustained community transmission n defined as the virus passing many times between people who are not part of the same family or circle of friends n has so far been documented only in Mexico and the US, a single WHO region. In Europe, infections have been limited to friends, close contacts and family members.

A surge in the number of cases in Spain, where 20 people were confirmed with the virus yesterday, is being monitored closely by the WHO.

Sustained community-wide transmission in there would quickly lead to a decision to move to phase 6, with full pandemic plans being rolled out in all countries, including difficult decisions on vaccination.

Over the weekend, further evidence emerged that the virus is spreading south and east. Hong Kong and South Korea reported their first cases, as did Costa Rica, the first case of H1N1 flu in Central America.