William Hill is the most recent victim in a series of cyber attacks that have affected companies such as Twitter, Netflix and the Guardian newspaper in the previous month.
The company apologised to their customers after their website crashed on Tuesday due to a “sophisticated” attack.
Hills stated that it had been the victim of a distributed denial of service attack, under which a target website is flooded with traffic so it cannot function properly.
The incident prohibited customers from placing bets on Tuesday evening’s UEFA Champions League action, including the matches involving Arsenal and Manchester City.
Considering the company’s H1 revenue of £814m, the 24-hour outage could have cost the publicly listed company roughly £4.4m, although the true figure is likely to be less as the attack only affected its online services.
A statement from the bookmaker on Wednesday said: “The online services of William Hill were intermittently impacted during the course of yesterday following distributed denial of service activity by third parties. This follows a significant increase in DDoS activity experienced by a number of online companies over recent weeks.
“While the attempt at disruption is ongoing our technical teams were able to restore services last night. We apologies for any inconvenience caused to our customers.”