Hospital cleaners and porters could go on strike next month after being offered a one per cent pay rise.
Unite said hundreds of its members employed by Serco at Barts Health NHS Trust in London had voted overwhelmingly for industrial action.
The union said unless there is an agreement, strikes will be set for January.
Unite said staff had been offered a 1% pay rise.
Unite general secretary Sharon Graham said: "It's time to take note of the strength of feeling of these NHS workers, who are being left in limbo as Barts and Serco prevaricate and pass the buck on their demands for a decent wage.
"Unite will be unwavering in our support of these union members and unless Barts and Serco put an end to their low pay and exploitation there will be strike action in January."
Shane DeGaris, deputy group chief executive at Barts Health NHS Trust, said: "Over the next 14 months we will be considering future arrangements of the facilities management contract, which could include bringing some services back in-house.
"We will be working with Serco to put the appropriate measures in place and ensure hospital services are supported during any strike action.
"We are working hard with Serco to see if strike action can be avoided".
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