Assembly Theatre will not be given first option to use the Assembly Rooms again and claims future use of the refurbished building during the festival will go to the "highest bidder".
Council leaders have admitting warning the long-running company that they will not get preferential treatment in August and say they have had strong interest from other promoters in taking on the A-listed building in George Street.
The local authority gave its final approval to a 9.3 refurbishment of the building on Thursday, with the Assembly Rooms set to lie empty during next year's festival while work is carried out. An open tender will be held to decide who will be allowed to stage Fringe shows, with a three-year contract expected to be up for grabs.
However Assembly Theatre has attacked the council for showing a lack of loyalty to the company, which has given many big-name acts their breakthrough at the venue.
Veteran producer Guy Masterson, actors Simon Callow and Brian Cox, and comedians Johnny Vegas and Jo Brand were among the stars to back a campaign against the council's plans, which will lead to the loss of five performance areas to make way for two shops and a restaurant.
Senior councillors and officials have been infuriated at the campaign orchestrated by Assembly's artistic director William Burdett-Coutts, who claims his company's fears over the impact the changes would have on the Fringe were ignored.
Sandy Ross, a director of Assembly Theatre, told The Scotsman: "We have no arrangement in place to use the building again when it is refurbished and we have been told it will be put out to open tender. We think it will simply go out to the highest bigger to run it during the Fringe.
"We've had a great relationship with the council for 30 years and that seems to have been forgotten about. We've certainly not had any guarantees about having first option for using it during the Fringe. We've simply been told the building will be let out according to the council's current policies and we will have to compete with everyone else.
"It's by no means certain we'll be able to use the building again. The Assembly Rooms as people know it will certainly not be back."
Council officials insisted a decision on who to let the building to during the Fringe would not simply be made on financial grounds.However it was claimed it has been receiving "serious interest" from other promoters.
Steve Cardownie, the council's festivals and events champion, said: "Assembly Theatre has had a very good deal over the years, but it is only right that the use of the building during the Fringe is put out to open tender.
"There is no contractual obligation for them to use the building again.
"I've been dismayed by the campaign that has been run by Assembly Theatre and but we need to put that past us now."