Britain’s most dangerous prisoner is locked underground in a glass box after issuing a chilling warning about plans to kill.

Robert Maudsley is a convicted serial killer who is currently serving a life in prison at Category A HMP Wakefield, widely known as ‘Monster Mansion’ due to the large number of high-profile and high-risk offenders held there.

Maudsley’s first murder came in 1974 when he garrotted John Farrell, who had picked him up for sex and showed him evidence of child sexual abuse.

After handing himself over to police saying he needed psychiatric help, Maudsley was sent to Broadmoor Hospital - where he and another prisoner tortured and killed child molester David Francis in Broadmoor Hospital. He was then transferred to HMP Wakefield, where he killed two inmates.

Channel 5

Maudsley has been behind bars in solitary confinement for the last 45 years in a specially built cell, which he once described as a ‘concrete coffin’.

He wrote in a letter: "The prison authorities see me as a problem, and their solution has been to put me into solitary confinement and throw away the key, to bury me alive in a concrete coffin.

"It does not matter to them whether I am mad or bad. They do not know the answer and they do not care just so long as I am kept out of sight and out of mind.

"I am left to stagnate, vegetate and to regress; left to confront my solitary head-on with people who have eyes but don't see and who have ears but don't hear, who have mouths but don't speak.

"My life in solitary is one long period of unbroken depression."

Details of the cell were also shared in Channel 5 documentary HMP Wakefield: Evil Behind Bars, which aired last year.

Channel 5

His nephew, Gavin Mawdsley, explained how the killer – who murdered three of his four victims while in prison – had made a chilling statement before being placed in solitary confinement in the specially-built glass cage.

He explained that, for two of the victims, Maudsley had declared he was going to kill as many rapists and sexual offenders as he could before getting caught.

Elsewhere in the programme, Mawdsley recalled the moment he found out the shocking details of his uncle's murders, having not known about the extent of the crimes until he spotted an article in the newspaper.

"I always knew I had an Uncle Bob, and I knew that he was in prison and that he’d done wrong," he explained.

"But I didn’t really know the details of his crimes."

Channel 5

At the time, press incorrectly reported that Maudsley had eaten part of one of his victim’s brains, and in turn he received the nickname of ‘Hannibal the Cannibal’.

He continued: “And it was when I was in senior school. There was a double article in one of the newspapers, and one of my friends was reading it, and said, ‘Hey, Gav, is this Hannibal Maudsley your uncle?’.

“And there was a picture of him, and one of the other guys said, ‘Bleeding hell, he looks like you’.

“And I knew straight away, ‘That’s my Uncle Bob’. And it was only once I read that article that gave me an insight into what he’d actually done, and how serious it actually was.”