A group of 15 Unison members will walk out for 48 hours tomorrow. They work for Mencap, a charity which provides staff for Wakefield Council at five small care homes for learning disabled people in the district.

But Unison claims vulnerable people could be distressed because of inadequate cover during the strike.

The dispute started when about 30 NHS staff were transferred to Mencap in 2004 on a higher pay scale than the existing Mencap staff. The charity decided to halt the new employees' annual pay increases to reduce the gap with original staff.

But Unison demanded that the charity continued to pay the rises, and that any unpaid rises should be back-dated to October.

Jim Bell, regional officer for Unison, said both Mencap and Wakefield Council had refused to engage in talks to resolve the dispute, which affects five small care homes for learning disabled people in the district.

He said: "Mencap's failure to engage with us means that the cover provided may be from people unknown to the residents and that in itself could cause them some distress."

But Mencap said fully qualified interim staff would be in place during the strike. And Wakefield Council insisted it had been willing to talk through the dispute, andwould work with the charity to provide any extra cover needed.

A Mencap spokesman said: "Extensive records are kept of all individual service users and we will be using experienced staff to ensure the continuity of care does not compromise the level of service received by people living in Mencap accommodation.

"We regret that Unison is taking strike action. Our priority is to continue to provide a quality service for people with a learning disability and our first concern is to make sure their service is not disrupted."

On Monday Mencap will no longer hold the contract with Wakefield Council, with services being taken over by a new provider called Choice Support. The new company plans to meet union officials in April in a bid to resolve the dispute.

Unison is still pressing forward with the action, saying it will still hold Mencap liable for the cash its members would have received under the pay rise.

A Unison protest will take place on Monday morning at Mencap's headquarters on Calder Vale Road, Wakefield.