Shares in 888 Holdings dropped sharply after the online gambling firm revealed it is being probed by the UK's betting watchdog.
Gibraltar-based 888 said the Gambling Commission had put one of its subsidiaries under review to ensure it was complying with "social responsibility" measures towards its customers. The update to investors saw shares tumble almost 8% in early trading.
The move comes after reports England and Manchester United striker Wayne Rooney lost £500,000 ($650,000, €590,000) in a late-night gambling spree, during which he spent two hours running up his bill on roulette and blackjack at a Manchester casino.
Also, last month Burnley midfielder Joey Barton was banned from football for 18 months after admitting a Football Association charge in relation to betting on matches. Neither case is known to be related to 888.
888 added that the probe by the Gambling Commission included a review of the use of "effective self-exclusion tools across different operating platforms".
Online firms are required to offer self-exclusion programmes to punters. This means a problem gambler can ask to be barred from a site for at least six months.
During that time, under Gambling Commission rules, the online betting firm must close the customer's account, returning any money in it. The business must also ensure that the customer's name and details are removed from any marketing databases it uses.
Online firms also offer 'time out' features, which mean that punters can block themselves from playing games for set periods of 24 hours, one week, one month, or up to a maximum of six weeks.
888 said it would work with the Gambling Commission in a "cooperative and collaborative manner" throughout the review, and would make further announcements "as and when appropriate".