The parties worked with Acas in an attempt to resolve the issue last week but the offer was rejected by 98% of GMB members, the union said.
Louis Bryson, GMB’s regional organiser, said: “Workers in our communities getting pushed towards poverty is a disgrace, especially such vital healthcare professionals.”
He said senior hospital managers were “penny-pinching the overtime rates of the lowest paid staff” while recruiting more senior management.
The union said about 70 porters at the hospital are members.
Dias added: “This is something we didn’t want [to do], We tried by all means not to come to this.
“It’s an investment in the future hoping that it will get better… The loss today could be a win tomorrow. We chose the loss today.”