According to BBC Sport Olympic Stadium bosses have been ordered to reveal details of their deal with West Ham United over the club's use of the east London venue.
The London Legacy Development Corporation (LLDC) fought a ruling that the contract should be made public, but the appeal has been rejected by an Information Tribunal.
The LLDC said the decision could cost the organisation "millions of pounds".
West Ham are due to move to the Olympic Stadium at the end of the season.
A LLDC spokesperson said the decision "could significantly impact [on] the stadium's ability to act competitively" and it is considering its next steps.
It has the right to appeal against the latest decision, but only on a point of law.
West Ham's vice-chairman Karren Brady has previously said she fought for the best deal, but has denied this was at the expense of taxpayers.
A fans' group which called for the publication of the contract said it was "naturally delighted with the outcome".
This latest decision marks a significant chapter in a long-running tug of war over whether the deal should be made public.
Supporters of publication say they have every right to know where public money has been spent.
The LLDC insists commercial sensitivity could impact on other business, while West Ham are keeping their distance, saying they have nothing to hide.
If the deal is published, the focus is likely to be on how much rent the Hammers are paying and the arrangements for a stadium naming rights partner.