A pathologist has cast doubt on alleged killer Stefano Brizzi's claim he accidentally strangled a police officer during a sex game gone wrong.

Brizzi, 50, has said in a defence statement that he met Gordon Semple on Grindr and arranged to have sex at his south London flat on April 1.

While there, they allegedly watched pornography and took crystal meth before Brizzi placed a hood over Mr Semple's head upon his request to be restrained and whipped.

According to Brizzi, the officer asked him to tighten a leash as much as possible, and when it momentarily slipped his neck "snapped", the Old Bailey heard.

Afterwards, the defendant has said he panicked having discovered the dead man was a police officer and took steps to dispose of the body.

Giving evidence, Dr Benjamin Swift told jurors there was evidence Mr Semple had been strangled as he had found a fracture to a tiny horseshoe-shaped bone in his neck, as well as bruising.

While he may have passed out after 10 or 20 seconds of pressure, it would have taken minutes for him to die, the pathologist told jurors.

During that time, the victim may have suffered fits as a result of a lack of oxygen to the brain and "stiffening" of the muscles, he said.

Dr Swift recorded the cause of death as "unascertained" because Mr Semple's body was dismembered and parts were never recovered.

The court heard he was presented with a number of plastic boxes containing body parts taken from Brizzi's flat as well as from the banks of the River Thames.

They included parts of Mr Semple's head, neck and shoulder blade, as well as a number of bones which had been stripped of skin and flesh.

One piece had an area indicating a possible bite mark, and there was also evidence of "charring" and contact with acid, the court heard.

Sharp bladed tools, including a saw, as well as a blunt object were used to break up the head and body, jurors were told.

The officer's brain and most of the internal organs remain unaccounted for, the court heard.

Brizzi denies murdering the 59-year-old officer but has admitted obstructing a coroner by disposing of the body.