LONDON — Russian criminals are making millions of dollars out of the H1N1 flu pandemic by selling fake flu drugs over the Internet, a web security firm said Monday.

Sophos, a British security software firm said it had intercepted hundreds of millions of fake pharmaceutical spam adverts and web sites this year, many of them trying to sell counterfeit antiviral drugs like Tamiflu to worried customers.

Sophos said many of the gangs behind the sites were based in Russia, and the top five countries buying fake Tamiflu and other medicines on the Internet were the United States, Germany, Britain, Canada and France.

Sophos spokesman Graham Cluley said a “worrying trend” toward stockpiling Tamiflu had already been seen in Britain, Europe’s worst-hit country.

Sophos said criminal gangs were operating medicines web sites branded as the “Canadian Pharmacy” to try to appear genuine. It said its research showed that on one network operated out of Russia, called Glavmed, it was possible to earn an average of $16,000 a day promoting pharmaceutical web sites.

“But the criminals can be members of more than one affiliate network, and some have boasted of earning more than $100,000 per day,” it said.