Perry was found dead at his home in Los Angeles, aged 54, on 28 October last year.

The actor had been taking ketamine six to eight times a day before he died, according to court documents.

Five people have been charged in connection with his death.

This includes his live-in personal assistant, Kenneth Iwamasa, a doctor named Salvador Plasencia, known as "Dr. P", Jasveen Sangha who was referred to in documents as the "Ketamine Queen", Eric Fleming, who it was claimed bought the drug off of Sangha, and then a second doctor Mark Chavez.

Chavez is due to plead guilty to one count of conspiracy to distribute ketamine when he appears in a federal court in Los Angeles, on Friday.

The 54-year-old from San Diego signed an agreement with prosecutors earlier this month.

Also working with the US attorney's office are Perry's assistant, Iwamasa, and Fleming - who both pleaded guilty to various charges.

The three are helping prosecutors as they go after their main targets, Plasencia and Sangha.

After a guilty plea, Chavez could get up to 10 years in prison when he is sentenced.

Meanwhile, Plasencia pleaded not guilty to one count of conspiracy to distribute ketamine.

Sangha, a dual US and UK citizen, pleaded not guilty to conspiracy to distribute ketamine, maintaining a drug-involved premises, possession with intent to distribute methamphetamine, possession with intent to distribute ketamine, and five counts of distribution of ketamine.

He and Sangha are scheduled to return to court next week.

They have separate trial dates set for October, but prosecutors are seeking a single trial that likely would be delayed to next year.

'I wonder how much this moron will pay'

When Perry died, the levels of ketamine in his body were equivalent to the amount used for general anaesthesia during surgery, according to Sky News' US partner NBC News.

In texts written by Plasencia, he wrote: "I wonder how much this moron will pay,", referring to Perry, prosecutors have alleged.

Court documents revealed extensive correspondence between some of the defendants as they discussed sourcing the drug for Perry.

In a separate message, Fleming allegedly sent a picture to Iwamasa of three unmarked jars said to contain ketamine with the message: "I guarantee it's going to be amazing."

They would use codewords as well and refer to ketamine as "Dr Pepper".

'Out of control'

Ketamine has seen a huge surge in use in recent years, as a treatment for depression, anxiety, and pain.

Perry had begun taking ketamine as part of infusion therapy to treat depression and anxiety, but, according to prosecutors, had developed an "out of control" dependence on it and began sourcing unsupervised doses.

Drug Enforcement Administration (DEA) administrator Anne Milgram said: "Matthew Perry sought treatment for depression and anxiety and went to a local clinic where he became addicted to intravenous ketamine.

"When clinic doctors refused to increase his dosage, he turned to unscrupulous doctors who saw Perry as a way to make quick money."

Prosecutors allege Chavez funnelled ketamine to Plasencia, securing some of the drug from a wholesale distributor through a fraudulent prescription.

In one instance, prosecutors allege that Plasencia "charged Perry $2,000 (£1,500) a vial that cost Dr Chavez approximately $12 (£9)".