In pictures: Severalls Hospital
Severalls Hospital in Colchester opened in 1913 and officially closed in 1997. However, a small number of the buildings are still in use by the NHS. This aerial picture was taken in 1999.
BBC Essex took these exclusive pictures after entering the grounds to see for ourselves the state of repair of the buildings.
William Palin, secretary of conservation group, Save Britain's Heritage, has described the current state of Severalls as: "The worst case of public sector neglect they've ever seen."
The team from BBC Essex came across widespread graffiti, vandalism and fire damage inside many of the buildings on the former Severalls mental hospital in Colchester.
Over 2000 patients were treated at Severalls Hospital at its peak and most of the site now stands derelict since it closed 12 years ago.
This picture was taken on Friday, 16, October and shows a piece of panelling in the outer ring of fencing having been removed, allowing easy access to the site.
The same piece of fence on Wednesday, 21, October showing repair work having taken place to the outer ring of fencing.
The Severalls Hospital site covers around 150 acres with many of the buildings in a derelict and dangerous condition.
Twelve years since the hospital closed the site is surrounded by secure fencing and has restricted access. This inner secure fence protects buildings in the centre of the site, including some with historic importance.
North Essex Partnership NHS Foundation Trust said only one part of the site has listed buildings on it and these include the water tower and main reception.