Staff at Bridlington and Scarborough hospitals have begun a strike over planned changes to their employment contract.
Porters, cleaners and estates team started their 48 hour walk out at 6am today.
Last week NHS Improvement, the body that oversees the trusts, said it would be consulting and issuing new guidance.
Trade union Unite says York Teaching Hospital NHS Foundation Trust wants to transfer the workers to a wholly owned subsidiary designed to avoid paying tax.
Unite says it is concerned that trusts are forming these wholly owned subsidiary companies in England so that they can register for VAT exemption and compete on a level playing field with commercial competitors who register for VAT exemption for their work in the NHS, when NHS trusts can’t.
Its lead officer for health in Yorkshire, Chris Daly, said: “We have received a very impressive mandate from our members for industrial action and, as a result, they will strike for 48 hours between September 27-29.
“The vote showed that our members wish to remain delivering vital public services – they are not prepared to let that public sector ethos slip away.
“If there is an emergency during the week of industrial action, our members will, of course, respond in a responsible fashion and put patient safety first.
“We are still 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.
“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."