William Hill reports record weekly loss in January
William Hill made a record weekly loss of £14m in January after a series of "customer-friendly" football results.
A high number of key Premier League and European matches were won by the favourite team in the third week of this year.
In that week, leading teams including Chelsea, Arsenal, Manchester United and Tottenham, and in Europe, Barcelona and Real Madrid, all won.
In lunchtime trading, the company's shares were down 3.3%.
William Hill also said it had extra costs of £20m from the introduction of a new "point of consumption" tax and a higher tax on gaming machines.
That helped to send operating profit for the 13 weeks to March down by 19% or £16m.
Betting in its 2,360 shops fell by 4%, but online betting rose by 16%, while mobile gaming rose by 48%.
Betting at the important Cheltenham Festival in March was up by 29%.
Over the year, the company closed around 100 shops and changed the way it staffs the outlets.
James Henderson, chief executive of William Hill, said: "Operating profit benefited from the changes we made to the staffing model in April 2014 as well as from the reduction in costs driven by shop closures".
He also said the betting firm had not yet made up for the football loss.
Earlier this year, William Hill had attempted to take over online rival 888 for more than £700m.
The offer was rejected and talks failed after what 888 called a "significant difference of opinion".