Paddy Power Betfair parent Flutter Entertainment said it would take a £90-110m hit to full-year earnings if coronavirus sports fixture restrictions remain in place until the end of August, but horse racing continues without spectators.
If horse racing is cancelled in Australia, Britain and Ireland and its shops are closed, Flutter estimated that this would incrementally reduce group earnings before interest, tax, depreciation and amortisation (EBITDA) by around £30m a month.
Shares in the company fell 20% at the opening of trade in Dublin this morning.
The Dublin-based group's EBITDA in 2019, excluding its investment in the US, was £426m.
It said today that before a raft of sporting cancellations, trading so far this year had been running ahead of expectations.
Major sports events cancelled or suspended due to the virus include elite soccer competitions such as the English Premier League and Champions League, as well as golf's first major of the year, the Masters, all big events for bookmakers. All GAA matches have also been cancelled.
Exceptions include some Australian sports - where Flutter operates under the Sportsbet brand - and British, Australian and Irish horse racing.
Britain's annual Cheltenham racing festival - a major event for Paddy Power Betfair - went ahead without restrictions last year.
Flutter agreed in October to buy Poker Stars operator Stars Group in a $6 billion share deal set to create the world's largest online betting and gambling company by revenue.
It generated around 78% of its revenues through bets placed on global sporting events last week.
Flutter said in its profit warning that it retained a strong balance sheet with a net debt to EBITDA ratio of 0.7 times at the end of 2019, well below its covenant level of 3.5 times earnings and that it will continue to explore ways to mitigate the impact of cancellations.
"The challenge currently facing our business and the industry more widely is unprecedented in modern times," Flutter's chief executive Peter Jackson said in a statement.
"While our near-term profitability will be impacted by the essential measures being taken globally, the board will remain focused on protecting shareholder value and managing the business through these turbulent times," the CEO added.
Flutter Entertainment's warning hammered shares across a gambling sector reliant on event fixtures for punters to bet on.
Flutter's big losses today dragged rivals William Hill 29% lower and Ladbrokes-owner GVC down 22% in London trade.
Sports now account for roughly half of William Hill's revenue after it closed 700 of its UK high street betting shops in 2019 over a government crackdown on slot machines.
They also made up around half of GVC's overall net gaming revenue.
Bookmakers are already under pressure from rising taxes in major markets and the crackdown in Britain which includes the use of credit cards for online betting.
They have responded with a push into the US market where rules have been relaxed.
Shares in Flutter saw big falls in Dublin trade today.