The bookie received 'big money' on the controversial scheme to scoop the UK's top architectural award on Saturday night (15 October).
As a result William Hill was forced to close its book on Monday.
However, according to a spokesperson, the move may have come 'a bit too late'.
Just moments before it stopped taking bets, William Hill was still offering impressive odds of 5/1 on Enric Miralles and RMJM's Holyrood building.
The spokesperson said: 'It would not be good for us if this came in. It would be the worst winner for us.'
If the Edinburgh landmark romps home at the weekend, it will be the second major prize it has landed in less than a fortnight.
Last Wednesday (5 October) the building was also named as Scotland's Best Building when it won the £25,000 RIAS Andrew Doolan Award.
But, if William Hill's closing odds are anything to go by, it will be either Foster's McLaren Technology Centre or the Jubilee Library in Brighton by Bennetts Associates and Lomax Cassidy & Edwards that will take the accolade.
Both were listed at 5/2 favourites - followed by O'Donnell + Tuomey Architects' Lewis Glucksman Gallery in Cork at 7/2 and Zaha Hadid's BMW Central building at 5/1.
Will Alsop's Fawood Children's Centre in London was rated as the rank outsider at just 7/1.