Over 140 workers at leisure centres in Wigan are set to strike over changes to pay.
Fitness instructors, swimming pool attendants and other staff at six leisure centres formerly run by Wigan council are balloting on whether to take industrial action.
Wigan Leisure and Culture Trust took control of the centres from the council three years ago.
Terry Mellor, the GMB union organiser, said the trust has now announced a new flat pay rate – removing rates paid for weekends and overtime.
Some workers could lose £3,000 a year.
Mellor said the workers had been sent letters informing them they would be dismissed on 5 May if they did not accept the new rates.
Mellor said, “Our members feel that we are being held to ransom. It is almost like blackmail.”