The little white bottle claims to hold 75 milligrams of oseltamivir phosphate - the generic name for the flu drug Tamiflu.

But consumers hoping their purchases over the Internet will help them survive a possible bird flu outbreak are being warned that rather than Tamiflu they might simply be buying vitamin C.

U.S. authorities this week seized 51 packages of counterfeit Tamiflu, a treatment for flu made by Roche Holding AG that governments have stockpiled to ward off deadly avian flu.

Counterfeit versions of Tamiflu have also now cropped up in Britain and the Netherlands, the Swiss Agency for Therapeutic Products, Swissmedic, said on Wednesday.

“Initial laboratory tests have shown that the products contained vitamin C instead of the active ingredient oseltamivir,” Swissmedic said in a statement.

The batches in question had been ordered over the Internet from suppliers in Asia, it said.

A spokeswoman for Roche in Basel confirmed that there had been one case reported in the Netherlands where someone bought a product falsely claiming to be Tamiflu on the Internet.

“The product came in a strange bottle saying generic Tamiflu,” the spokeswoman said.

However, while Roche has entered into talks with various southeast Asian countries and companies about producing generic versions of Tamiflu, no officially sanctioned version of the drug has yet been made.

Swissmedic said that no bogus Tamiflu had made its way into the official supply chain - something Roche says would be very unlikely anyway, given the security features included in its packaging.

Britain’s Medicines and Healthcare Products Regulatory Agency said it had identified around 20 Internet sites that were illegally advertising Tamiflu, four of them in the UK, and was analysing test purchases from the sites for bogus ingredients.

Roche has repeatedly warned consumers not to buy the medicine over the Web, not least because it requires a prescription from a doctor.

An Internet search throws up scores of sites advertising generic Tamiflu, alongside drugs purporting to be copycat versions of impotence treatments Viagra and Cialis, as well as the sleeping pill Ambien.