Shares in William Hill plunged more than 13% early on Wednesday (23 March), after the British bookmaker warned that its annual profits will be lower than last year.
The FTSE 250 group said profits at its online division could fall between £20m ($28.4m, €25.4m) and £25m as an increasing number of punters used time outs features, which allow customers to lock themselves out of their accounts.
The bookmaker also reported the number of online customers opting for an automatic self-exclusion period from online activity was on the rise, warning that if "these trends persist" the group's annual profit will be significantly lower.
The London-listed company forecast that its operating profit for 2016 as a whole will be between £260m to £280m, below the £291.4m reported last year. However, group chief executive James Henderson said the company had enjoyed favourable football results in the UK and was striving to improve its online experience.
"We continue to focus on improving online's performance so that we can, once again, outperform the market," he said, adding that last week's Cheltenham Festival was the worst result in recent history.
On the first day of the four-day festival, the bookmaker lost £2m after favourites Annie Power, Douvan and Vroum Vroum Mag all ran out comfortable winners.
Meanwhile, William Hill has also confirmed it was in advanced discussion to invest in gaming software company Openbet. However, the bookmaker added the negotiation "may, or may not, lead to a transaction".