A 7,000 signature petition over the future of the QE Leisure Centre in Wimborne is to be debated by Dorset councillors this week.
The petition calls on the council to stop the closure of the centre.
It says the decision in 2022 by the former Conservative-controlled council to end its subsidy within two years could eventually lead to the school having to find at least £850,000 each year if public use at the centre, now run by community groups, is to continue.
The figure assumes it will no longer received an existing government grant of £296,000 from 2025.
The QE School Foundation Trust failed to find an operator to run the centre but has allowed continued public sessions where they are run by community groups, paying to hire the facilities they need.
From 2019 Dorset Council pumped almost £3 million into the centre to meet the revenue deficit for public sports and leisure.
A public consultation, started by the council in the autumn of 2021 on its intention to withdraw from its agreement with the Trust, found that there were eight alternative public leisure centres within a 10-mile radius.
As a result of the consultation the council decided to withdraw from the dual use management agreement at the end of March 2024, with council officers continuing to help the centre maximise its public use.
It has also offered up to £150,000 towards the replacement of an all-weather pitch.
The petition will be presented to a council meeting on Thursday evening by campaigner Steve Parlour.
He will be given three minutes to speak and present the petition with the council then able to debate the issue for no more than 15 minutes.