Increasing amounts of spam offering drugs to deal with bird flu have been reported across the world's email systems.

Security firm Sophos is reporting a large increase in emails offering online purchases of Tamiflu, the drug manufactured by Roche that is the only known medicine for dealing with the human version of avian flu.

But according to Roche, which has investigated similar scams, it is unlikely that the drugs on offer are real.

"Spammers are not interested in people's health, they're interested in making fat profits," said Graham Cluley, senior technology consultant at Sophos.

"Drugs like Tamiflu should be prescribed by legitimate doctors, not quacks on the internet. Buying medicine online, from a website advertised by spam email, is like playing Russian Roulette."

The British Medical Association confirmed that it does not advise anyone to purchase drugs of any type over the internet.