Admin and clerical staff at Mid Yorkshire Hospitals Trust will walk out in protest at plans to impose pay cuts of up to £2,800-a-year.
Bosses at cash-strapped Mid Yorkshire, which runs Pinderfields, Pontefract and Dewsbury hospitals, want to downgrade the salaries of staff including medical secretaries and receptionists to help save £24m.
Unions say the plans unfairly target low paid and mainly female members of staff, and have already staged four days of strike action.
Now notice has been served of a further five-day strike from midnight on Sunday.
Jim Bell, regional organiser for Unison, said: “This is a significant escalation of the action from our perspective. We expected around 500 people to be on strike during the week.”
Trust bosses offered a deal to increase the period staff would be protected from the pay cuts from a year to 18 months and give them six months pay protection in a lump sum.
But Mr Bell said Mid Yorkshire had already cut pay protection from three years to a year before proposing the current downbanding of salaries.
He added: “It’s a cynical move by them.”
Graham Briggs, Mid Yorkshire’s director of HR, said the trust remained open to meaningful discussions to resolve the dispute.
He said: “We must reduce our costs and in particular our pay bill and it is disappointing that our continued attempts to work with union representatives to find a mutually agreeable solution for the trust and the members they represent continue to be rejected.”