Porters, cleaners and caterers at the hospital were outsourced to private company ISS during the pandemic, but were taken back in house in April 2023.
The timing of the transfer means that 60% of them have missed out on the lump sum payment given to directly employed NHS staff and colleagues on Agenda for Change linked contracts.
The workers have previously taken 18 days of strike action in the dispute.
Kerry Nash, GMB senior organiser, said: ‘These workers put their lives on the line during the pandemic - just like other NHS staff.
‘It's a disgrace they have not been paid the covid bonus they so richly deserve, just because the trust had outsourced the service.'