While the lowest paid NHS workers received a £2,000 pay rise last year, those on private contracts received nothing, according to Unison.
Low-paid workers like cleaners, porters and security guards on private contracts are leaving the health service in search of better paid jobs, which is affecting the “smooth running” of the NHS, the union said.
Unison, which has 1.3 million members, called on the government to make sure everyone working in the NHS earns at least £9.03 an hour.
Currently a worker on minimum wage (£8.21 an hour) would earn £1,600 less each year than those on the NHS’s lowest rate of pay (£9.03).
Sarah Gorton, head of health at Unison, said: “All hospital workers are part of the NHS team and should be paid fairly for the important jobs they do. The days of treating them as second class employees in the NHS must end.
“Staff employed by private contractors are expected to deliver the same exceptional levels of services and also work under immense pressure. It’s only fair they receive the same pay as colleagues, often doing identical jobs but employed by the NHS.”
Gorton said that ministers should not let Brexit distract them from the “growing crisis” in UK hospitals and should find the money to fund a pay rise for health staff on private contracts.
“A failure to do so risks damaging the health service beyond repair as firms can’t attract or hold on to the staff needed to provide a decent service to the NHS,” she said.
Separate analysis by the Labour party and the House of Commons library has found that a total of 21 NHS contracts worth £127m are currently out to tender.
Of these, 19 contracts worth £36m have been put out since mid-February.
The health and social care committee recently heard that NHS procurement rules that require contracts over a certain value to be put out to tender can be a “huge waste” of time and money.
