Russian business people and officials have stolen up to $30bn from funds intended for preparations for next year's Sochi Winter Olympics, according to a report released by opponents of President Vladimir Putin.

Putin, who has staked his reputation on a successful Games, faces criticism over allegations of corruption and costs overruns that have pushed up the price tag for the event to $50bn - five times more than initially planned.

"The main conclusion from the first chapter of our report is that, in preparing for the Olympics, $25bn to $30bn were stolen," Russian opposition leader Boris Nemtsov told reporters on Thursday.

The report, written by Nemtsov and Leonid Martynyuk, another Putin critic, alleges that the most expensive sports complexes built for the Games were commissioned without competition or public tenders.

Government officials have previously dismissed reports of widespread corruption around the Games and have defended the increased costs.