Flutter Entertainment Plc, which operates the Paddy Power and Betfair brands, estimated that it would face a 90 to 110 million pound impact on full-year earnings if curbs on sports fixtures remained in place until the end of August.
That figure was based on horse racing continuing but only behind closed doors, as is the plan in Britain.
GVC Holdings Plc, owners of the Ladbrokes betting shop chain, said its full-year core earnings may be reduced by 130-150 million pounds because of fewer sports fixtures.
That calculation was based on the soccer season remaining on hold until July, the Euro 2020 tournament being delayed for a year, and major horse racing meetings such as Aintree and Royal Ascot being cancelled.
William Hill suspended its dividend and warned of an impact of 100-110 million pounds on core earnings if its British high street shops had to close for a month and most sports events remained suspended until August.
Shares in William Hill plunged 34%, GVC lost 20% while Flutter's London-listed shares fell 12%.
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Flutter generated around 78% of its revenue through bets placed on global sporting events last year.
Sports accounted for 53% of William Hill's revenue in 2019, while GVC generates around 45% of its core revenue from sporting events.
Major sports events suspended or cancelled due to the virus include elite soccer competitions such as the English Premier League, Spain's la Liga, Germany's Bundesliga and the Champions League, as well as golf's first major of the year, the Masters, all big events for bookmakers.
Ulrik Bengtsson, the chief executive of William Hill, said the company had taken bets on horse and greyhound racing, the international soccer that was still being played and virtual sports in recent days.
Bookmakers are already under pressure from rising taxes in major markets and a crackdown in Britain which includes limits on the size of stakes on gaming machines in betting shops. They have responded with a push into the U.S. market where rules have been relaxed.
"The challenge currently facing our business and the industry more widely is unprecedented in modern times," Flutter Chief Executive Peter Jackson said in a statement.
Flutter agreed last October to buy Canadian Poker Stars operator Stars Group Inc (TSG) in a $6 billion share deal set to create the world's largest online betting and gambling company by revenue.
(Additional reporting by Patrick Graham and Tanishaa Nadkar in Bengaluru, editing by Emelia Sithole-Matarise and Keith Weir)
By Padraic Halpin and Tanishaa Nadkar