A man has pleaded guilty to stabbing his grandmother to death in a frenzied attack at their family home in Mitcham.

Schizophrenic Kordian Filmanowicz had been living with his 63-year-old grandmother Ilona Czuper, along with his mother and sister in Edgehill Road since 2016 after they became worried about his mental ill-health.

But shortly before midnight on July 6 last year, Ilona's body was found in the back garden of the home by her daughter and granddaughter.

She had been repeatedly attacked and the bodies of the family's pet cat and dog lay nearby.

The Old Bailey heard there were no witnesses to the attack but the evidence against Filmanowicz was "overwhelming."

In the lead-up to the murder, he had refused to take his medication and began behaving erratically.

CCTV footage showed him walking away from his grandmother's house around 8.25pm that day. It is believed he attacked her late afternoon as neighbours heard a noise, although did not realise what was happening.

At the hearing on July 23, the 21-year-old pleaded guilty to manslaughter on the grounds of diminished responsibility.

Detective Chief Inspector John Massey, from the Homicide and Major Crime Command, said: "This has been an incredibly sad case, the manslaughter of a beloved grandmother at the hands of her grandson while he suffered an episode of severe mental ill-health.

"The CCTV footage and forensic examination of the scene proved conclusively that Filmanowicz was responsible for this brutal and horrific attack."

Forensic examination of the scene showed Ilona had been attacked outside close to the house and had run down the garden trying to escape Filmanowicz. The animals were killed inside and dragged into the garden.

Filmanowicz will be sentenced on Tuesday, July 31 at the same court