The Western Trust is being called on to "do more" to save the Gransha Hospital site in Derry.
It comes after 35 firefighters attended a large fire which was first reported around 7:30pm on May 8.
It's understood fire crews from the city and Co Derry brought the blaze under control in the early hours of Monday morning.
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A spokesperson for Northern Ireland Fire and Rescue said: “Four Fire Appliances, one Aerial Appliance, one Water Tanker and a Command Support Unit and 35 Firefighters attended the incident.
"The crews were from Crescent Link, Northland, Limavady, Dungiven and Kilrea Fire Stations.
“The incident was dealt with by 2.01am on Monday 9 May 2022 and the cause of the fire is believed to be deliberate ignition.”
The former Stradreagh Hospital, which used to be an asylum before it was used to accommodate a local school, was damaged in another deliberate fire in 2016.
The Ulster Architectural Heritage said it was "disappointing" to see the hospital in its current state.
A spokesperson said: "Very disappointing to see that there was a fire at the former Stradreagh/Gransha Hospital in Londonderry last night. This hasn't been the first time that the building has been deliberately damaged by fire.
"The hospital was built in stages from 1897-1933. The buildings were recently listed and added to the heritage at risk register last year."
Meanwhile, Enagh Youth Forum called on the Western Trust to do more to save the building."
"This historic building has laid vacant for many years now and for far too long. Gransha and Enagh areas are beautiful areas.
"There's been numerous fires at the site for the past ten years."
MyDerry has contacted the Western Trust for comment.
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