Experts in London said they are not surprised that a strain of drug-resistant swine flu has emerged among people with underlying health problems.

Professor Peter Openshaw, a respiratory physician at Imperial College London, said the five patients at the University Hospital of Wales in Cardiff who were diagnosed with Tamiflu-resistant swine flu would have been more susceptible because their immune systems were not working properly.

He said: “It’s not surprising that this has happened, indeed it has always been anticipated.

“The situation in Wales is to do with infection among immunosuppressed patients who are always more likely to develop resistance.”

He added: “The important thing now is to emphasise the value of vaccination. just because there is resistance to Tamiflu does not mean there is resistance to (anti-viral drug) Relenza/Zanamivir.”

Dr John McCauley, of the Medical Research Council’s National Institute for Medical Research, said: “The resistance in Wales is in patients who are an at-risk group. It is well established that patients with immunodeficiency do have problems with eliminating virus, giving it a greater chance to acquire resistance. Full genetic sequencing may be able to discover more detail.”

Another expert called for urgent action to improve defences against infections such as swine flu, warning that drug-resistant strains needed to be contained before they become widespread and more virulent.

Dr Ron Cutler, an expert in infectious disease control and deputy director of Biomedical Science at Queen Mary, University of London, said: “These recent developments with flu virus demonstrate that there is a definite need to improve our defences against these viruses.

“Shortening the time taken to produce new vaccines and improving the methods to control and treat the disease while vaccines are being made would be a way forward.

“This could be achieved by developing new methods to improve infection control, to reduce viral transmission and spread, and by increasing the numbers and variety of antiviral agents to control and kill these viruses.”