Robin Williams was clear of alcohol and illegal drugs when he killed himself, tests have revealed.

Postmortem examination results released by Marin County sheriff’s office found that the actor had taken prescription medication, but in “therapeutic concentrations”.

The coroner ruled Williams’ death was suicide resulting from asphyxia due to hanging.

Police said Williams (63), was found in the bedroom of his home in Tiburon, Northern California, on August 11th. His wife, Susan Schneider, has said the actor and comedian was struggling with depression, anxiety and a recent Parkinson's diagnosis when his personal assistant found his body.

Williams had publicly acknowledged periodic struggles with substance abuse and he had entered an abuse programme shortly before his death.

According to the coroner’s report, his wife told an investigator Williams did not go there because of recent drug or alcohol abuse, but rather to reaffirm the principles of his rehabilitation.

According to his wife, Williams was having trouble sleeping and had shown increased signs of paranoia before he took his own life, the report said.

Medical records confirmed he was diagnosed with Parkinson’s disease in November 2013 but had symptoms since 2011, including a left arm tremor and the slowing of left hand movements. Treatments with drugs in May led to some improvement, according to the report, and he remained physically active until his death.

Williams had two types of antidepressants in his system when he died, as well as a Parkinson’s medication, caffeine and another ingredient found in tea and cocoa, the tests found.

Authorities have said Williams was last seen alive by his wife when she went to bed on the night of August 10th.

She woke up the next morning and left, thinking he was still asleep elsewhere in the house.

Shortly afterwards, Williams’ assistant came to the home and became concerned when he failed to respond to knocks at a door. The assistant found the actor in a bedroom.