Microsoft's online advertising platform has been found wanting just days after inking a lucrative search and advertising deal with Yahoo, after a new report claimed that rogue internet pharmacies are advertising on its Bing search site.

The No Prescription Required report (PDF), released by anti-spam company KnujOn and online verification firm LegitScript, said that nearly 90 per cent of the sponsored pharmaceutical adverts found on Bing "led to 'rogue' internet pharmacies that do not require a prescription for prescription drugs, or are otherwise acting unlawfully or fraudulently".

The report also alleged that some of the adverts appeared to be for a legitimate US-based internet pharmacy, but clicked through to a different, illegal pharmacy site.

Some of these rogue online pharmacies are actually fronts for Russian organised crime gangs, according to KnujOn and LegitScript.

The authors said that they intend to release similar studies naming and shaming "US-based corporations that facilitate, and/or profit from, 'rogue' internet pharmacies".

A Microsoft spokesperson said the firm is "currently investigating the issue ".

"Microsoft’s guidelines clearly require online pharmacies who advertise on Bing to adhere to US laws," the spokesperson added.