If given the go-ahead, the move would affect 44 workers, who are members of the union Unite, who are employed by the private contractor Serco. Angry staff claim they are missing out on the same pay and benefits that there NHS employed counterparts receive. They are campaigning for the company to introduce a system similar to the NHS's Agenda for Change scheme, in which wages were reconfigured.

Mark Robinson, Unite regional officer, said: “Our members did not ask to be transferred to a private company when the hospital moved from its city centre site.

“All our members want is to be treated and paid the equivalent as if they worked in other directly employed NHS trusts such as Norfolk PCT and Norfolk and Waveney Mental Health Partnership.

“Whilst the company's proposals come close on pay, they remain significantly short on annual leave provisions.”

Mr Robinson said many of the workers were forced to transfer to contractors after years of service as NHS employees when the hospital moved from the city centre site to Colney.

Ballot papers were sent out to Unite members on Wednesday and voting will close on October 8.

Martin Payne, contract director for Serco, said: “Our primary concern is for the patient, but we've been talking to the union for the best part of six months to agree a pay and conditions package which is comparable to the Agenda for Change scheme.

“We've been able to do that in terms of pay and gave a 2.5% rise in April and we think we've made a fair and equitable offer in terms of the pay part of it.

“We've been trying to come to a resolution and now we are just waiting for the outcome of the ballot.”

The hospital has no direct involvement in the pay talks.