Robin Williams spent his last days in a paranoid frenzy, aware that ‘something else was wrong with him’, a British television show will claim tonight.

In a ‘peculiar’ incident just one day before his death, the Oscar-winning actor, who had been diagnosed with Parkinson’s, stuffed his collection of watches into a sock and took it to a friend for safekeeping.

The 63-year-old, who had also been prone to episodes of insomnia and anxiety prior to his death, also spent his final 24 hours frantically looking up drugs online, convinced he had another illness.

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The post-mortem examination later revealed that Williams had been suffering from Lewy body dementia – an undiagnosed illness would could have been the root cause of his bizarre behaviour.

In tonight’s episode of Autopsy, which will reconstruct the final hours of the actor’s life, pathologist Dr Richard Shepherd explains how Williams’ dementia probably triggered his paranoid tendencies.

He says that Williams’s insomnia and anxiety, as well as periods of confusion and impaired decision making, could also have been connected to the condition.

But Dr Shepherd also tells the Channel 5 show – which has previously examined the unexpected deaths of Michael Jackson and Elvis Presley – that the actor may well have been aware of his undetected illness, something which could have exacerbated his paranoia in his final hours.

He says that Williams’s online activity suggested he knew ‘there was something else wrong with him… something that he couldn’t put his finger on’.

Williams hanged himself in August 2014, a death which sent shockwaves through the celebrity world and led to an outpouring of grief from his worldwide fans.

His death was later ruled to have been suicide. The show reveals tonight how, on the day he died, Williams cleaned his bathroom of blood after cutting his wrists.

According to the post-mortem report, Williams spent his last night at his Californian home with wife Susan Schneider.

Ms Schneider later described how her husband stuffed a number of his jewelled watches into a sock before driving to the house of a friend.

Speaking about the incident on tonight’s show, Dr Shepherd says: ‘This is a very peculiar incident that his wife describes as typical of the increasing paranoia that he had been suffering.’

Psychologist Anjula Mutanda also says the actor could have felt ‘under threat’.

‘Somebody experiencing paranoid feelings may fear that they are in danger and under threat,’ she says. Whether it is physiological, physical or financial – harm could be coming their way.’

After Williams’s body was found, a bottle of quetiapine, a powerful drug used to treat schizophrenia and bipolar disorder, was also found unopened in his bedroom.

He had been prescribed it a week before his suicide, suggesting his symptoms may have changed prior to his death.

In the months before his death, Williams had also checked into rehab to ‘fine-tune and focus’ on his commitment to staying clean.

But the documentary concludes there were no signs that drink or drugs were exacerbating his depression when he died.

Blood samples taken after his death showed he had not taken cocaine or alcohol in the last 24 hours of his life. There was also no sign of damage to his liver from previous drinking binges.

The show also describes how toxicology reports showed Williams’ moods were being controlled by mirtazapine, which is used to treat depressive disorders.

There was also levodopa in his system, which is used to treat Parkinson’s.

The forthcoming documentary, part of the third series of the show, has already drawn criticism from Williams’s fans after it emerged that it featured a ‘graphic’ reconstruction of the actor’s suicide.

A Channel 5 spokesman previously said the depiction was kept to ‘an absolute minimum’ but that its inclusion ‘is important in telling the truth’.

They added: ‘The film celebrates Robin’s career and talent, and within that the tragic details of his death are part of that story.’

French actor Alain Poudensan plays Williams in the reconstructed scenes. While Alain has worked as a Robin Williams impersonator for a number of years, he has also starred in many adult films, under the name Alain L’Yle.

Williams found fame with his portrayal of a kooky alien in the 1970s sitcom Mork and Mindy.

But it was his role as an irreverent DJ with the US Armed Services Radio station in Good Morning, Vietnam in 1987 which won him huge acclaim.

His roles ranged from serious and dramatic in films such as Dead Poets Society and Good Will Hunting, to comedy in Mrs Doubtfire.

He was nominated for an Oscar three times before winning an Academy Award for his performance as a psychologist in Good Will Hunting in 1997.

Mrs Schneider Williams, his third wife, is currently in a battle with his three children from previous marriages – Zak, Zelda and Cody – over his £33million estate.

Zelda recently spoke out about her father’s death. When asked why she believed her father took his own life, she said: ‘I don’t think there’s a point. It’s not important to ask.’

• Autopsy: The Last Hours of Robin Williams airs tonight on Channel 5 at 9pm.