Top professional poker player Phil Ivey won big over two August nights at Crockfords, an exclusive casino in the Mayfair district of London.
But the casino, the oldest in Britain, is refusing to cough up the cash according to media reports.
Crockfords is reportedly probing the $11.5 million win by the 35-year-old Californian pro and winner of eight World Series of Poker bracelets.
Ivey, reports the Daily Mail, struck a "remarkable winning streak" while playing Punto Banco - a no-skills-needed variation of baccarat.
He was accompanied by a "beautiful woman" and played over two nights during the Bank Holiday long weekend of August 25 to 27 for approximately seven hours. His companion's membership at another Mayfair casino had at one time been suspended, it is alleged, and this may have raised the casino's concerns.
While it is reported that the casino initially agreed to transfer the winnings into the poker champ's bank account, only Ivey's "£1 million stake" has been delivered, reports the Mail.
A female croupier and Crockfords staff have been interviewed after the casino allegedly feared Ivey was in collusion with the casino's workers, reports the Mail.
While the full details are not known, it is believed that Crockfords has ruled out collusion between Ivey and their staff.
Cardplayer.com reports that no evidence for cheating has been uncovered, and that Ivey has not been accused of anything.
The police are believed to not be involved, while lawyers, reports the Mail, are "engaged in an increasingly tense stand-off".