Service and security staff in Suffolk and north Essex hospitals will strike later this month after the trust's privatisation.
In a ballot of UNISON members, 99% voted in favour of strike action in response to a review of the provision of services being carried out by East Suffolk and North Essex NHS Foundation Trust (ESNEFT) across its sites.
The strikes will affect Colchester Hospital and community sites such as Aldeburgh and Felixstowe hospitals and the rehabilitation clinic at Bluebird Lodge in Ipswich.
It will take place between Monday, August 19 and Friday, August 23.
The union took this decision as ESNEFT announced its intention to "outsource" services to include cleaners, porters and caterers at all of its sites, a move which could affect 4,022 workers.
Cleaning and security services at Ipswich Hospital, which is also run by ESNEFT, are currently provided by private company OCS - but its contract comes to an end in April next year.
Following this, the NHS trust wants to outsource the services across all of its sites.
Nick Hulmes, chief executive of ESNEFT, said: “We are disappointed to learn that Unison has gone down this route while we are still actively involved in ongoing conversations with them and their members.
"We are listening to our staff about the issues that are important to them."
UNISON says staff fear the move will threaten their pay and conditions and pose a serious risk to patient safety.
The union also added that outsourced staff in Ipswich currently get fewer days of annual leave and less sick pay than their colleagues directly employed by the trust.
Additionally, it said Ipswich staff also missed out on the extra one-off payment of £1,655 that NHS staff received in the last financial year.
UNISON Eastern regional organiser Sam Older said: “These workers are proud to work for the NHS and support patients. They don’t want to strike but feel they’ve been left with no choice. Their bosses simply won’t listen to their concerns.
“It’s very easy for ESNEFT to prevent these strikes. Trust managers just need to abandon their nonsensical plan to sell staff out of the NHS."
Sarah Gray, a facilities team leader in Aldeburgh, added: “Staff should be respected just like other NHS colleagues. Our pay grade and what we do every day should be no different from other health workers at the hospital.”
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