Nearly 50 staff are to strike at Croydon's leisure centres on Thursday over pay.
Fitness instructors, swimming teachers and customer assistants are among the 48 employees who will walk out over the dispute with Fusion, who run Croydon's five centres.
The union Unite claimed the strike could force the centres to shut, but Croydon Council said it had been assured they would remain open.
Unite accuses Fusion of cutting costs by employing a "multi-tier" workforce in which staff doing the same jobs earn varying rates of pay and have different basic terms and conditions.
It said the company's contract with Croydon Council was awarded in 2007 on the basis staff were employed on a single-tier basis.
Onay Kasab, Unite regional officer, said "The borough’s leisure centres face closure on Thursday and this debacle at Fusion is an example of what happens when councils privatise services.
"We call on Croydon Council to intervene and tell Fusion to start treating its workers with fairness, not contempt."
The strike will last from midnight on Wednesday until midnight on Thursday.
Unite have warned further industrial action could take place at Croydon's leisure centres and at other across London.
A spokesman for Croydon Council said: "Fusion have assured the council the centres will remain open largely as normal."