The world tennis authorities have admitted they took no action to pursue 28 top-level players, including winners of Grand Slam titles, named in a secret 2008 report into suspected match-fixing. All were allowed to continue playing and a third were flagged up again after playing in more suspicious games. The revelation is contained in a cache of documents obtained by the BBC and BuzzFeed from tennis insiders.

For years the annual tennis tournament in Sopot, a pretty coastal town in Poland, had passed off almost unnoticed. There was little TV coverage and even images of the tournament were rare. But in August 2007 that changed, when Nikolay Davydenko, then ranked number four in the world, took on the Argentine player, Martin Vassallo Arguello, ranked well below him.

Davydenko should have been the overwhelming favourite, but something strange was happening to the betting on the game. By the start of the match he was no longer the favourite, and even after he won the first set and led the second, big bets continued to be placed against him.

Mark Davies, managing director of the online betting exchange, Betfair, had just left for a meeting with the Cabinet Secretary when his phone rang. “It was our legal director,” he says, “and he said to me, ‘We have got the worst tennis match that we have ever had on the site.'”

A group of Russian gamblers had staked millions of pounds on Betfair that Arguello would win. When Davydenko retired and forfeited the match Betfair took the unprecedented step of voiding all bets on the game.