The report, by Trinava Consulting, claims that most of the work done by subcos comes from the NHS trusts that set them up and much of the profit is generated from squeezing staff pensions.

Report author, Vivek Kotecha, said: ‘Despite running for many years, few subcos have won any significant external customers. Their profits come mainly through skirting around tax and employment rules.

‘Likely changes to VAT rules and restrictions on altering workers' terms and conditions put the future of these profits in doubt and there's no reason to encourage NHS trusts to set new ones up.'

Unison says the growth of subcos has been an increasingly controversial feature of the NHS landscape, with trusts using them as a mechanism to make savings by driving down pay and pensions for new staff.

In September last year, the government announced a policy change on subcos. Ministers said any future transfer of NHS workers into subsidiary companies would be approved only where there was clear union support for the move and NHS conditions were protected.

Unison wants to see staff at existing subcos brought back into the NHS, where they will be guaranteed access to a better pension and other benefits, such as enhanced development opportunities.

Christina McAnea, Unison general secretary, said: ‘This report explodes the myth subcos offer anything other than a way to deny staff the same pay and conditions as NHS workers.

‘Any profit is courtesy of contracts with the trusts that originally set them up and claims of efficiency are simply down to a VAT loophole that's likely to be closed anyway.

‘Farming staff out to subsidiary firms undermines the principle of a single NHS workforce and creates unnecessary anxiety for employees.

‘The NHS relies on support staff to keep hospitals clean, safe and running efficiently. They deserve to be treated like other health workers.

‘Subcos have never been the right way to deliver health services and it's time to return staff stuck in subcos back into the NHS.'

The report, Assessing the finances of NHS subsidiary companies, was funded by Unison's Campaign Fund.