A Carnforth man who was locked in a cell with serial killer Harold Shipman has spoken of his horror at hearing the doctor boast of mass murder.

Jon Harkin, a Samaritan-trained ' listener' who was locked up overnight with Shipman to ensure the disgraced doctor did not commit suicide, was aghast as Shipman described his murderous deeds as 'acts of God.'

'Doctor Death' has never publicly admitted his guilt, given an indication of how many people he killed or offered a motive.

But during gruelling 'listener' sessions, Shipman told fellow inmate Harkin that he had taken hundreds of lives over 25 years simply because he had the 'power' to do so.

Harkin's account gives a disturbing insight into the mind of Shipman, who has revealed nothing to the police or authorities.

Many believe the disgraced doctor would take his secrets to the grave but Harkin has decided to break 'listener' confidentiality in this exceptional case to share his experiences with the families of the victims.

"I have had to sit through the trial, the appeal and the inquiry knowing the terrible truth of what he told me," says Harkin, who kept suicide watch at Preston Prison where Shipman was held on remand.

"My time listening to Shipman was horrendous.

I've been physically ill and my memories of being locked in a cell with him have given me many sleepless nights.

I'm just glad he was found guilty and is unlikely to ever be released from jail.

But I believe the families of those people he killed have a right to know what he said to me inside that prison cell."

Dame Janet Smith's Shipman Inquiry concluded in June 2002 that the family doctor from Hyde had unlawfully killed 215 people between 1975 to 1998.

After examining 2,311 witness statements she said: "I regret to say that I can shed very little light on why Shipman killed his patients."

But Harkin remembers vividly how Shipman, filled with 'contempt and arrogance', boastfully claimed to have taken the lives of more than 500 people.

Describing the doctor as being on a 'crazed power trip' after being formally charged with multiple murder, Shipman bragged to his fellow inmate that the actual numbers were far higher.

"He was so full of himself, so arrogant, as if he was on a pedestal, laughing at the police and the medical profession.

I asked him why he took their lives.

He kept saying that he had the power, that they were ' acts of God.' It wasn't a confession - it was more like a boast."

Harkin had taken great pride in developing the listener scheme and reducing the self-harm and suicide rate at Preston Prison.

As senior listener, he offered support in a crisis suite to more than 300 inmates and was given free access to all wings.