Croydon Council has agreed a deal to sell the former Ashburton Library building.
Three bids to buy the property, in Ashburton Park, were lodged with the council after it went on the market in August.
The identity of the winning bidder and their proposed usage for the building remains unknown.
But the council had pledged to look to ensure the building remains "relevant for and accessible to the local community".
More than 20 parties had expressed an interest in purchasing the building, formerly a convent, but only three made firm offers.
One of the bidders is understood to have been an evangelical church based in Thornton Heath.
Friends of Ashburton Park, a community group formed last year, had hoped to use 'right to bid' legislation to pause the sale.
The law is designed to afford registered community groups time to prepare a bid for important local buildings, but the group failed to get the former library reclassified.
The long leasehold sale, subject to approval by the council's corporate services committee on Wednesday night, would give the successful bidder a 125-year lease on the building.
A report containing details of the sale has not been made public.
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