The World Health Organisation is investigating whether a form of swine flu resistant to antiviral drugs being stock-piled in Britain is spreading from person to person.

The strain has been identified in a 16-year-old girl who was stopped by officials at Hong Kong airport after falling ill. She had arrived on a flight from San Francisco.

The symptoms were mild and she was released from hospital after a week. However, subsequent tests revealed she was carrying a form of the virus that was resistant to Tamiflu, the main drug used to combat swine flu.

Two other patients, in Denmark and Japan, have been identified with a Tamiflu-resistant strain of the virus, but they developed it after being treated with the drug, indicating that it had mutated.

Crucially, the girl in Hong Kong had not been given Tamiflu, suggesting that she had picked it up from another person. Health officials are now trying to trace everyone she had been in contact with.

The development of a widespread resistant variant of swine flu would limit treatment options before a vaccine becomes available. In Britain, the government has stock-piled 30m courses of Tamiflu and is giving the drug to every patient with the virus.

Health authorities are investigating whether the Hong Kong type developed after swapping genes with existing seasonal H1 flu strains, almost all of which are resistant to Tamiflu.

Dr Nikki Shindo, a medical officer with the World Health Organisation, said: “The Hong Kong case is different because this person hasn’t seen any treatment.

“She has been identified at the airport so we suppose there are a considerable number of contacts in the aeroplane.” John Oxford, professor of virology at Queen Mary, University of London, sounded a note of caution. He said: “At this stage it is just one case. The predominant strain of swine flu remains susceptible to Tamiflu.”

Andy Burnham, the health secretary, has warned that fresh cases of swine flu could top more than 100,000-a-day by the end of next month.

Last week a man aged 19 from south London became the fourth person in Britain to die after contracting the virus, although he had underlying health problems.

The actor Rupert Grint, who plays Ron Weasley in the Harry Potter films, is recovering from a “mild bout” of swine flu, his publicist said yesterday.

Grint, took a few days away from filming the latest Potter instalment, but is now able to return to work.

“Scenes were shot in his absence which did not require his involvement and thus filming was not disrupted,” his publicist said.

“He is looking forward to joining his fellow cast members at the premieres this week and will then return to filming directly afterwards.”

Grint’s doctor has confirmed that the actor is no longer contagious.

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