The controversial victory by Enric Miralles and RMJM's Scottish Parliament building has cost bookmaker William Hill a 'five-figure sum' and reinforced fears about 'suspicious betting patterns'.
The bookie approached the RIBA to discuss whether the result could somehow have been let out of the bag before the awards, a scenario the RIBA vehemently denies.
Rupert Adams of William Hill said: 'After the result we had a long chat with the RIBA. It is far and away the worst loss ever for us on the Stirling Prize.'
Just days before the event, the bookmaker said it received a number of 'big money' bets on Holyrood - at the time offered at odds of 5/1 - raising fears about insider knowledge. William Hill then stopped taking bets.
Adams added: 'Our compilers study 50 different markets and they spotted some suspicious patterns here.
'The Stirling Prize got flagged up straight away. It just didn't look right and the compilers got nervous - it was they who closed the betting.'
The RIBA's head of awards, Tony Chapman, is adamant the result could not have been leaked prior to the live announcement. He said: 'There couldn't have been a fix.
'The award was sorted out over three hours on the Saturday. The process was extremely rigorous and the judges batted it back and forth before they took a secret ballot.
He added: 'I really have no idea why there was a sudden run of money on the Scottish Parliament. I assume a lot of Scots backed their favourite.'
Piers Gough, who was on the Stirling judging panel, believes the bookmaker simply made a mistake and was offering odds too good to be ignored. He said: 'William Hill's odds were in the exact reverse order to the ones I had them in. It's quite comical really.
'William Hill got it very wrong. If they don't want to get it wrong again they should visit the buildings. They should also not be surprised when a building of such genius wins.'
He added: 'The decision couldn't have been made before Stirling. I had not seen the other judges and even Jack Pringle said he didn't know the winner before he opened the envelope.'