William Hill rejected a second takeover approach worth £3.2bn ($4.1bn, €3.7bn) from gambling groups Rank Group and 888, saying their offer "continues to substantially undervalue" the business.
Britain's largest bookmaker said the new proposal offered its shareholders an estimated value of 352p a share, compared to last week's offer of 339p a share.
It added that the bid "presents significant risk for William Hill shareholders as it involves a highly complicated three-way combination at a very low premium."
William Hill said the fresh deal was a 12% premium to the bookies' undisturbed share price before the first bid by its rivals was announced. It added it saw "no merit" in holding talks with the bidders.
Rank Group and 888 said last week their bid offered "a compelling value creation opportunity for William Hill".
But William Hill chairman Gareth Davis said the offer was "highly opportunistic and complex and does not enhance the strategic positioning of William Hill". It added it would continue its plan of building its online and international divisions. William Hill has operations in the US and Australia.
Last month the bookies chief executive, James Henderson, stepped down with immediate effect, as the industry struggles under tougher gaming machine regulations and online taxes.
The gambling 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 last year agreed to merge, 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.