Staff represented by the union, who work as porter drivers, delivering samples, medication, linen and food across the Manchester region, will walk out after voting unanimously for industrial action.

Unison says the workers had taken part in a job evaluation process, which found they should be moved to a higher pay grade. However, the union says the trust has not acted on the findings, costing staff thousands of pounds in lost income they should be rightfully paid.

Ben Jackson, Unison Greater Manchester Mental Health branch secretary, said: ‘The trust has moved the goalposts for staff. Managers decided to step outside the nationally recognised process because they didn't like the outcome. The workers feel extremely let down.'

Unison North West regional organiser, Paddy Cleary, added: ‘Staff had already agreed the details of their back pay and future pay progression, only for the trust to change lanes at the 11th hour.

‘This is outrageous behaviour for a trust aiming to rebuild its reputation. It's not too late to do the right thing, pay staff fairly and avoid this strike.'

In response, Marc Reed, director of capital, estates and facilities at Greater Manchester Mental Health NHS Foundation Trust, said: ‘Robust plans are in place to minimise disruption to deliveries to and from prioritised services, and the industrial action will only involve driver porters within GMMH's central Transport department, based at the Prestwich Site.

‘We remain in active discussions with Unison and local portering representatives to address the issues being raised and continue to work constructively to resolve the dispute.'