Staff at the former Calderstones Hospital site will be holding a rally on Monday demanding answers over job security.
Health workers with Mersey Care, at the Whalley site, are seeking answers from the NHS trust over their futures.
Trust chiefs say learning disability services at the Mitton Road location will be transferred to Rowan View Hospital in Maghull, as part of long-term Mersey Care proposals to withdraw from Whalley.
Secure unit patients have been transferred away from the old Calderstones site over the course of several years and other services closed.
Chris Chamley, union branch secretary for GMB Mersey Care, said: “The site has been retracted a lot now, areas have been shut down. The old (medium secure unit) was shut down, another ward area has shut down.
“Now there is no definite thing happening with staff at the Whalley site. There is talk of Lancashire Care taking over the buildings in October, but this would just be the buildings and not the staff.”
Bosses at Mersey Care first took over the former Calderstone NHS Partnership in 2016, which then employed 800 across Lancashire and Greater Manchester. A government review, led by Sir Stephen Bubb, published that December, recommended the closure of the Whalley site.
Mr Chamley added: “Staff have had enough, staff are leaving, and we just need answers, when we try and get answers its all ‘well this may be happening, that may be happening’, there is nothing guaranteed.
“That doesn’t give staff job security and I think it’s really unfair. We want to know our future and want to know what it holds for us.”
Up to 50 staff could attend the ‘peaceful’ rally at the Whalley site, he said.
A spokesman for Mersey Care said: “Our learning disability services at the site are due to close in the near future following a public consultation guiding a decision made at national level in 2016.
“Mersey Care puts the highest priority on the welfare of our staff and we have been working closely with NHS England to keep staff and patient groups informed at every stage of the relocation of our services to a state of the art centre of excellence for forensic care at Rowan View Hospital.
“We expect to provide services from Whalley for the next year before handing over to Lancashire and South Cumbria NHS Foundation Trust in autumn 2023, who will deliver different healthcare services.
“Mersey Care values the work of our staff at Whalley and would like them to stay within our organisation, and we are looking at providing incentives to enable them to work from other trust sites.
“Mersey Care appreciates some of our workforce may not wish to travel for work or have other commitments which prevent that and we have begun discussions with other local trusts to try and ensure the NHS retains their knowledge, skills and experience.”