Hundreds of hospital staff in Gloucestershire could be taken off the NHS's books in a bid to save money.
Gloucestershire Hospitals NHS Foundation Trust wants to set up a private company to employ about 900 members of its support staff.
The trust hopes a subsidiary company would improve services and reduce costs by reducing its VAT bill.
A union said it was a "risky tax dodge" and new staff might not have access to the same NHS terms and conditions.
The trust is currently in special financial measures and expecting to end the financial year £23.9m in debt.
The subsidiary company would be wholly owned by the trust but would see support staff - such as cleaners, porters and caterers - taken off the NHS payroll and employed by the company instead.
The trust's chief executive, Deborah Lee, said it could save the trust £5m a year.
"This has formed part of our strategy to reduce our costs without adversely impacting on the quality of services or requiring us to reduce the workforce," she said.
But unions say it is a first step towards privatisation.
There are also concerns that new members of staff will technically no longer be NHS employees, and will have new terms and conditions - affecting their sick pay, holiday allowances and access to an NHS pension.
'Want to stay'
Michael Sweetman, from the Unison union, said working for the NHS was a "badge of pride" for staff and the "900 affected want to stay in it".
"Warm words from the trust about the 'NHS family' won't mean much when sick pay, pensions and more are on the table," he said.
"We need joined up services where management focus on delivering care, rather than distracting themselves with risky tax dodges."
The trust is due to hold a consultation with staff before a formal decision is made.