Porters and cleaners at Scarborough and Bridlington Hospitals are being balloted about potential strike action.
Unite says York Teaching Hospital NHS Foundation Trust has set up a subsidiary companies which will employ the staff, so it can register for VAT exemption and compete on a level playing field with commercial competitors.
The union, which has 200 members employed in the estates and maintenance departments at the trust, claims the move could lead to job losses.
Its lead officer for health in Yorkshire, Chris Daly, said: “We are strongly against the formation of these entities which, we believe, could lead to a Pandora’s Box of Carillion-type meltdowns – with the adverse knock-on effects on patient services and jobs.
“Our members consider that the identity of their employer is a condition of their contract of employment and do not wish that being changed unilaterally, as is proposed in this case.
“They have a strong desire to remain employed by the trust and not to be employed by this subsidiary - York Teaching Hospital Facilities Management LLP – which has been set up with the purpose to avoid tax.
“We are seeking an undertaking from York Teaching Hospital NHS Foundation Trust that it agrees to continue to employ all our members and not transfer them to this new subsidiary with all the negative connotations that will flow from that decision.”
The ballot, which asks members if they support strike action, or industrial action which is short of a strike, opened yesterday and runs for a week.
Unite said pressure from health unions has seen trusts in Leicester and Wigan back down on their plans for a wholly-owned subsidiary company.
A spokesman for the trust said: “The LLP is wholly owned by the NHS as staff have told us that this is important to them. Staff moving to the new organisation will keep their current NHS terms and conditions.”