A mother with a "history of mental illness" strangled her seven-year-old son then drowned him in the bath, a court heard.

Christina Acres, 37, accepts that she killed George Acres at the family home in Rochford, but has pleaded not guilty to his murder by reason of insanity.

Judge Mrs Justice Christina Lambert told jurors at the start of the trial at Basildon Crown Court that two expert psychiatrists "appear to agree that Christina Acres is not guilty of murder by reason of insanity".

She said the case had come to trial as: "Only a jury can return a special verdict of not guilty by reason of insanity".

Andrew Jackson, prosecuting at Basildon Crown Court, said: "On any view, this is a deeply tragic case."

He said Acres killed George by strangling and then drowning him on the morning of July 23 last year, at the beginning of the school summer holidays.

Acres had a "history of mental illness" and her mental health deteriorated "quite rapidly" in the month prior to the killing, Mr Jackson said.

Acres had previously smoked up to 20 cannabis cigarettes per day to self-medicate for fibromyalgia, a long-term condition that causes pain all over the body, Mr Jackson said.

Acres and her son were living at her parents' home in Rochford at the time of the killing, following the breakdown of her marriage in 2012.

Symptoms of her psychotic illness "had begun again to manifest" themselves three days before the killing and her mother took her to A&E, Mr Jackson said.

They were advised to go to their GP for a prescription of anti-psychotic medication, the prosecution said.

"She had told her father at that time that she believed people were out to kill her and her family, and she displayed other disturbing thoughts," said Mr Jackson.

The court heard she also talked of the Illuminati, and that after the killing she referred to George as a "witch".

The trial, estimated to last four to five days, continues.