Unison is warning there could be further walkouts by NHS staff across the country if the government doesn't clamp down on hospital trusts farming out support services to subsidiary companies, known as SubCos.

The union reports that 94% of the workers, including porters, caterers and cleaners, backed strike action over their employers' plans to move them from the NHS to a new company.

The trusts – Dorset Healthcare University, Dorset County Hospital, and University Hospitals Dorset – want to move more than 1,700 mainly low-paid support workers to a SubCo.

The union is bringing a motion on SubCos to be debated at the Labour Conference, which begins in Liverpool on Sunday, to bring to the fore the promises the party made to reverse outsourcing.

The motion says proposals by health trusts for greater use of SubCos are a direct contradiction of the party's pre-election pledge to bring privatised public services back in house. The union says without prompt action, there could be widespread strike action across the NHS in the autumn.

In addition, Unison has written to all NHS trusts highlighting the dangers of setting up SubCos and reminding senior managers of campaigns fought and won by unions the last time these companies were used by desperate health bodies.

Unison general secretary Christina McAnea said: ‘Before the election, Labour promised to bring workers on private contracts back into public services. They need to make good on that promise and quickly, or they risk seeing widespread disruption in the NHS in coming months.

‘The overwhelming vote by Dorset support staff demonstrates just how strongly health workers feel about any erosion of their pay and conditions.

‘The NHS depends on support staff to keep hospitals clean, safe and running efficiently. Many are already low-paid and farming them off to subsidiary companies will leave them with an even worse deal.'

In response, chief executive of University Hospitals Dorset Siobhan Harrington and chief executive of Dorset County Hospital and Dorset HealthCare Matthew Bryant, said: ‘The three NHS trusts in Dorset are working together to improve the care we provide to our patients and to make the best use of our resources. We must stress that no final decision has been made on whether to go ahead with the proposed creation of this company. However, if we do, we would again stress this would be 100% owned by the NHS and we are committed to retain NHS pay, terms, conditions and access to the NHS pension scheme for current and future staff.

‘We are keen to avoid industrial action and remain committed to working formally and informally with unions and our staff to listen to and address concerns and anxiety. Our priority is minimising disruption to staff, patients and services.'