Cleaners, porters, catering assistants, security guards and other health staff employed by private companies should receive the same pay rise as NHS staff, union leaders have said.
The government has announced a 3% pay rise for most NHS workers in England.
But Unison said those working in the NHS but employed by private contractors were at risk of missing out.
It is calling for companies running health contracts to offer their staff the same increase.
The union said private firms and subsidiary companies owned by individual NHS trusts should increase hourly pay to at least match new NHS rates.
Unison has written to the largest private employers in the health service saying their staff “have worked hard and under incredibly challenging circumstances across the NHS throughout the pandemic”.
General secretary Christina McAnea said: “NHS staff have the benefit of a national pay system, but those not directly employed are missing out, often because of complex contracting arrangements, penny-pinching practices and the hard-nosed pursuit of profit.
“Staff in the NHS work on one site as one team, from maintaining clean and safe wards to ensuring patients are fed and cared for. No-one delivering NHS services should be paid less than their directly-employed colleagues.”
One cleaner, who works at an NHS hospital in Bedfordshire, told the BBC she was previously directly employed by the NHS and her income had gone down since the contract was handed over to a private firm.
There was deliberate obfuscation over payment terms “to make you confused”, she said, adding: “There is nowhere to go and complain. You can do nothing.”
The woman said that she and fellow colleagues “don’t know where we stand” with regard to the 3% pay rise being offered by government.
“Some say we are going to get it, some say we won’t. For me, I don’t believe it – I will wait until I see the money.”
Last month, the government offered NHS workers in England, including nurses, paramedics, consultants, dentists, salaried GPs, porters and cleaners, a 3% pay rise.
The level of the increase was criticised by unions, who said it did not reflect the sacrifices made by staff.
NHS staff in Wales have also been offered a 3% increase and in Scotland most have been offered a 4% pay rise.