William Hill said the group's FY operating profit for 2016 is about £260m, at the bottom end of its guided £260-£280m range.
In the nine weeks since the trading statement on 14 November 2016, wagering trends continued in line with those previously reported.
"However, gross win margins were below expectations, in large part due to unfavourable football and horseracing results impacting the sector during December," the company said in a statement.
Interim CEO Philip Bowcock commented:
"Importantly, the improvements we saw in wagering in Online and Australia in the second half have continued in recent weeks.
"However, all four divisions saw customer-friendly results at the back end of the year, which translated into profits being about £20m below our prior expectations.
"With key underlying trends continuing to be positive, the recent run of sporting results have not changed our confidence in a better performance in 2017."