Voluntary sector organisations are being used as a "Trojan horse" to break up the NHS, according to trade union Unite.
The union cited NHS East of England, NHS Cambridgeshire and Hinchingbrooke Health Care NHS Trust, which last month announced plans to invite third sector, private and NHS organisations to bid for the franchise to run Hinchingbrooke Hospital, near Huntingdon, as an example of the practice.
Staff will remain employed by the NHS and the hospital and its assets will still be owned by the NHS.
Rachael Maskell, Unite's national officer for the not-for-profit sector, said charities were being lured into a market place for contracts that made them "completely vulnerable" financially and compromised their ability to advocate for improvements.
"We cannot put lives at risk in what could be an unstable market place," she said. "Charities are the Trojan horse for privatisation."
Owen Granfield, Unite's lead officer for health in the eastern region, said both the union's health and its not-for-profit sector members were opposed to the break-up of the NHS and the fragmentation of services.
"It is clear that the NHS in the east of England is using the region as a guinea pig for various privatisation initiatives, such as social enterprises," he said.