The staff taking action are employed by a private company

Their campaign is set to be stepped up to include strike action in a dispute over pay.

Unison says the workers, who are employed by a private contractor Carillion, are on worse pay and conditions than NHS staff.

The union's regional officer, Jim Bell, said: "Our members are not prepared to exist on poverty pay as low as �4.25 an hour and are not prepared to continue working without the benefit of an occupational sick pay scheme.

"They have less holiday than colleagues doing the same job but directly employed by the NHS."

Two days of strike action are planned for later this month.

A spokeswoman for Carillion said the company still wants talks to find a "constructive outcome" to the dispute.