Bookmaker William Hill said punters winning £20m ($24m) in December on football and horseracing results meant its full-year estimates will come in at the bottom end of expectations.
The firm blamed unfavourable results at the end of last year for its annual operating profit estimates coming in at around £260m, the lower end of its £260-280m range.
The betting group said the Boxing Day football programme was the worst weekend on record with 18 out of the top 20 best-backed teams winning.
It added that horseracing during the Christmas period was also bad for bookies. A large number of favourites won races including the King George VI Chase, won by favourite Thistlecrack and the Welsh Grand National two days later, also won by favourite Native River.
The country's biggest bookie, which axed its chief executive in the summer after lacklustre trading, said these results meant its "gross win margins were below expectations".
Interim chief executive Philip Bowcock said: "All four divisions saw customer-friendly results at the back end of the year, which translated into profits being around £20m below our prior expectations."
Bowcock, was previously the group's chief financial officer, before former boss James Henderson left the group with immediate effect in July.
Bowcock added the group experienced improved trading at its online and Australian divisions in recent weeks, adding he had "confidence in a better performance in 2017".
Mergers in the sector
In November the group had said its trading would put it at the top end of its £260-280m guidance.
The company pulled out of merger talks with Canadian online gambling company Amaya in October,
A £3.2bn offer for William Hill by rivals Rank Group and 888 also collapsed in August after the market leader twice rejected their bid as undervaluing the business.
However, the sector is going through a period of consolidation that threatens the dominance of William Hill, which runs more than 2,300 outlets in the UK.
Betfair and Paddy Power agreed to merge in 2015, online gambling business GVC bought rival Bwin. Party and Ladbrokes and Gala Coral are also combining.
The sector is also in the middle of costly upgrades to its online betting platforms, the fastest-growing part of the industry.
Analysts at Numis said: "This is the third warning in 12 months and focuses us once again on William Hill's near term shortcomings, including UK retail exposure, losing market share online and the
lack of a permanent chief executive."