The soap star served 10 years of a life sentence for murder at HMP Kingston, off Milton Road, before he was released in 1976.
The former prisoner would go on to study at drama school and become a household name by playing serial-adulterer Den Watts.
Now, an urban explorer and paranormal investigator has visited the prison and claims to have encountered the actor’s spirit.
Spook-hunter Karl Hassall told the The Sun on Sunday newspaper: ‘We heard what sounded like Dirty Den’s voice saying ‘release them’, and ‘ search them’ – I asked ‘is there anyone here with us now’ and got a reply of ‘yeah’.’
Grantham was imprisoned after the killing of a cab driver during a botched robbery in Germany in 1966.
He landed the role he would turn into ‘Dirty Den’ in 1985, playing the unlikeable pub landlord for three years before leaving the show in 1989.
The small-screen legend died in 2018, aged 71.
HMP Kingston is set to become a 260-home development, with work to transform the site beginning last January.
It was closed by The Ministry of Justice in 2013 and sold in 2014.