European public health specialists on Monday identified significant resistance to the drug Tamiflu, casting a shadow over the efficacy of the world’s most widely purchased influenza antiviral medicine.

The European Centres for Disease Control said that while Tamiflu could still provide benefits, 13 per cent of samples of the H1N1 seasonal flu virus affecting Europe tested last November and December – most in Norway – contained a mutation associated with high levels of resistance.

The preliminary data, based on 148 samples collected from 10 European countries, are the first important indication of resistance to Tamiflu, which has been purchased in large quantities by governments around the world over the past three years as a defence against a flu pandemic.

Previous studies have indicated far lower resistance, with only about 0.4 per cent of seasonal viruses rendering Tamiflu treatment less effective. There have been four cases of drug resistance among the patients with current variants of H5N1 treated to date. The strain has infected 353 people worldwide since 2003.

The news is a blow to Roche, the Swiss pharmaceutical group that markets Tamiflu, and has turned the drug into a blockbuster largely on the back of stockpiling by governments as they prepare for a pandemic.

It also highlights the danger of pandemic planners focusing too heavily on medical preparations, and the need for other broader measures designed to identify, limit and curb the spread of infection.

David Reddy, responsible for Tamiflu at Roche, said: “This is a very new finding and better understanding is required of its relevance to a pandemic. The vast majority of flu viruses have shown susceptibility to Tamiflu.”

Separately, the European medicines agency has stressed the importance of pandemic planners stocking different antiviral drugs, balancing Tamiflu with Relenza, an alternative medicine less widely tested but which has so far not demonstrated resistance.

GlaxoSmithKline, the UK-based pharmaceutical group that markets Relenza, welcomed the guidance on Monday.

Roche has sold 220m treatments of Tamiflu in recent years for pandemic flu stockpiles to 85 countries.