A policeman did not die accidentally during a drug-fuelled sadomasochistic sex session, a pathologist said today.

Crystal meth addict Stefano Brizzi, 50, allegedly murdered and then chopped up PC Gordon Semple after they met through gay dating app Grindr.

The Italian social worker claimed Scottish-born PC Semple, 59, died after a leash around his neck momentarily slipped during a sex game, a jury was told.

But a pathologist told the jury pressure would have had to be applied for minutes not seconds as claimed by Brizzi.

In his defence statement, Brizzi claimed the officer had visited his south London flat and they took crystal meth and poppers, watched porn and had sex.

He claimed PC Semple wanted to be restrained, so Brizzi put a hood over his head and tied a leash around his neck to pull on while sitting on PC Semple's face.

He added they were interrupted by someone knocking on the front door, but PC Semple indicated he wanted him to go away.

Crispin Aylett QC, prosecuting, said: "A third party knocked on the door.

"The defence statement suggests Mr Semple was still alive, conscious and able to motion with a finger to turn the third party away.

"It was during this time that the deceased asked the defendant to tighten the collar as much as possible.

"Whilst tightening the leash, it apparently slipped."

Brizzi claimed he heard a snap, and realised PC Semple was unconscious and had no pulse, it was said.

In the statement he said he panicked, and became paranoid after searching PC Semple's clothes and discovering he was an officer, so took more drugs and started to get rid of the body.

But Benjamin Swift, a pathologist who carried out a post-mortem on PC Semple, said the pressure on the neck would have needed to be applied for longer to have killed him.

Less than 20 seconds, "potentially" less than ten seconds, could result in unconsciousness, he told the Old Bailey.

Dr Swift added: "However, release of the pressure on reaching unconsciousness will result in the individual awakening rapidly.

"In order to result in death, the force needs to be maintained beyond the point of unconsciousness - potentially minutes - to result in oxygen starvation of the brain.

"It's possible the deceased might have suffered fits during this time as a result of a lack of oxygen to the brain."

He told the court the cause of death was "unascertained" because of the extensive dismemberment of the body.

Dr Swift also described how body parts were delivered to him in plastic boxes to examine, but the brain has never been recovered.

The flesh had mostly been stripped from the bones, jurors were told.

Dr Swift said: "It was mostly the bones and the ligaments holding bones together.

"There were saw marks and cut marks, suggesting a bladed weapon had been used on the bone."

Turning to the head, he said there was evidence of "blunt force trauma" to the skull.

Dr Swift said: "There were fragments of skull still missing upon conclusion of the examination. The brain was absent.

"As well as a fragment of a back part of the skull, the nose bone and the bone behind the nose had been cut across."

Body parts which had never been recovered included most of the ribs on the left side, the left upper arm, the majority of the internal organs and most of the skin.

The neck was bruised, consistent with PC Semple having force applied to the neck, the court heard.

Dr Swift said: "This force may be in the form of manual gripping - manual strangulation - of in the form of pressure applied with an item such as a ligature."

Jurors have been told Brizzi may have cooked and eaten parts of PC Semple, as DNA was recovered from chopsticks and a kitchen pot.

Forensic scientist Cathryn King confirmed to the court today there was a one-in-a-billion match of PC Semple's DNA to the pool of fat and grease in the oven.

The oven door handle was blood stained, and the bottom of the oven contained a pool of fat and grease, jurors heard.

Mr Aylett said: "You took a sample from that pool of fat and grease.

"And that sample was submitted for DNA analysis, and it contains a complete DNA profile of Gordon Semple, with a billion to one match probability."

Mrs King confirmed that it did.

Diluted blood stains were also found in the bath, on the tiled bathroom walls, and above the toilet and sink, jurors were told.

There were also blood stains on the hall carpet, kitchen floor and work surfaces, it was said.

Mr Aylett said: "You concluded that attempts had been made to clean up the blood staining at different locations in the flat."

Mrs King also told the court there was a "one in a billion" DNA match of PC Semple on a perforated metal sheet and a wire brush.

Drops of blood were also found on the carpet of the bedroom.

Brizzi, of Southwark, south London, denies murder but admits obstructing the coroner in the execution of duty.

The trial continues.