Private contractors working in the NHS should disclose how much profit they are making, the union Unite claims.
The trade organisation said commercial confidentiality means that few details are available to evaluate if private companies are providing value for money.
It believes the only government figures available on non-NHS provision of healthcare is £4.1bn for 2004–05, which accounts for just 5.6% of the total NHS spend for that period.
Derek Simpson, Joint General Secretary of Unite, said: "There is a complete absence of evidence and rigorous evaluation of private contracts operating in the NHS and billions of pounds of public money is being spent and many more billions of pounds of profits earned.
"As well as undermining the public ethos of the NHS, this creeping privatisation is sucking the lifeblood out of public health care in Britain and depriving the NHS of vital funds.
"We want real and effective monitoring of NHS private contracts to establish whether or not they are providing value for money, delivering efficient services and offering the best in patient care. We also want to know exactly what profits private companies are earning from NHS contracts.
"Until these things are established, no further private sector contracts should be awarded in the NHS and the plan to open up the commissioning of services to the private sector should certainly be halted."