One of Marilyn Monroe's doctors covered up his role in the actress' death by denying he prescribed her one of the drugs that killed her, a biographer has claimed.

Monroe, famed for her roles in Gentlemen Prefer Blondes and In How to Marry a Millionaire, died in 1962 at the age of 36 from a barbiturate overdose inside her Los Angeles home.

The Los Angeles county coroner ruled her death as a probable suicide after she ingested a fatal combination of chloral hydrate, a sedative, and Nembutal.

Now, Hollywood author Andrew Wilson, 58, has claimed he has proof that Monroe's death - which has been subject to widespread conspiracy theories - should be viewed as a case of medical negligence at the hands of her doctor, Hyman Engelberg.

Engelberg, the movie star's personal physician, had maintained until his death in 2005 that he gave her Nembutal, but not the chloral hydrate.

The drugs, used to treat insomnia, a condition Monroe suffered from, should not be taken in conjunction.

Engelberg was interviewed in 1982 by the Los Angeles police after the force opened an investigation into Monroe's death following the emergence of conspiracy theories.

He claimed he knew 'nothing' about the chloral hydrate, and 'never used it'.

He said the 'only' prescription that he wrote the actress was that of Nembutal.

However, a prescription order from 1962 for chloral hydrate - which was sold at auction in 2011 - was found to have been signed by Engelberg.

The prescription was found by Wilson when conducting research for his book, I Wanna Be Loved By You: Marilyn Monroe, A Life in 100 Takes.

The book, which was published this year, comes ahead of the one hundredth anniversary of Monroe's birth.

This evidence, Wilson argues, proves Engelberg's role in her death was dishonest.

Speaking to The Times, he questioned that if Engelberg did not have anything to hide, why would he 'deny it so emphatically?'

He posited that in the weeks leading up to Monroe's death, Engelberg had been distracted due to his separation from his wife, Esther, leading him to 'panic'.

Wilson added that, if Monroe's death had been today, her doctor might have faced involuntary manslaughter charges for over-prescribing medication to a vulnerable patient.

He drew comparisons between Monroe and Friends actor Matthew Perry, who passed away aged 54 from an overdose.

Erik Fleming, a drug addiction counsellor who sourced the surgical anaesthetic ketamine that killed Perry, was one of five people charged in connection with his death.

Wilson said: 'It was an accident, but also a catastrophic slip of judgment.

'In the last two months of her life, she was prescribed 830 units of medication - enough to kill several people several times over.'

On the night of Monroe's death in Brentwood, her psychiatrist Ralph Greenson had been contacted by her housekeeper, Eunice Murray, who was alarmed that Monroe had locked herself in her bedroom.

Greenson, arriving at her home, broke the window and found the actress unalive on her bed.

He proceeded to call Engelberg, who pronounced Monroe dead.

Engelberg, however, did not call the police until nearly one hour later - and told officers Monroe had committed suicide.

The Strange Death of Marilyn Monroe, a 1964 pamphlet written by right-wing polemicist Frank A Capell is widely regarded as the source of conspiracy theories surrounding Monroe's death.

However, Wilson said he wanted to look at her death in a 'forensic manner'.

'This is a death scene but is it also a crime scene?' he said.

Capell argued that Monroe did not die by suicide but rather suggested she had been murdered as part of a communist conspiracy surrounding Robert F Kennedy, with whom she had been having an affair.

The idea was later picked up by author and journalist Norman Mailer in his 1973 best-selling biography of Monroe.

This publicity, Wilson argued, caused the conspiracy to be passed 'down the generations' - citing Monroe's ongoing connection to the Kennedys in the present.

Wilson, who has spent five years researching his book, said he wished to 'penetrate' beyond the accepted image of Monroe, as there has been 'nonsense' written about her, including the conspiracy theories.

He said he wished to frame Monroe, best known as the comic 'blonde bombshell' as a 'survivor', rather than a victim.