An alleged sadomasochist accused of murdering a policeman has told a court that the officer died while they were playing "a very dangerous game".

Stefano Brizzi, 50, described how Pc Gordon Semple, 59, had been enjoying the sex session at the Italian's home in south London on April 1 but then stopped breathing.

The pair, who met on the gay dating app Grindr, had been performing bondage actions, the jury at the Old Bailey heard.

Brizzi said Pc Semple was wearing a collar and a mask while he held a leash which he tightened at the policeman's request.

Standing in the witness box, Brizzi said he was "pulling the leash and counting until 30. I was following his instructions."

"I released the lead when I got up. I immediately untied his collar when I realised that he wasn't answering."

He said he was "calling him" and asking "what's going on".

Brizzi - who broke down in tears several times as he gave evidence - said he then removed Pc Semple's mask and "realised that he was completely unresponsive and I started to panic".

He slapped Pc Semple's face to try to wake him, gave him mouth to mouth and attempted chest compressions but to no avail, Brizzi told the court.

"Unfortunately I couldn't see any response. I also checked his pulse to see if I could feel any pulse. I realised that we had been playing a very dangerous game."

Brizzi thought about calling an ambulance but did not do so because he thought Pc Semple may have a "condition" such as asthma.

He said he checked Pc Semple's pocket to see if he had an inhaler but instead found his police badge. "I was absolutely petrified," Brizzi said.

Asked by Sallie Bennett-Jenkins QC, defending, to confirm that he did not call an ambulance, he replied: "Yes, I accept that."

Brizzi also told the court that he and Pc Semple had used code to indicate whether they were enjoying the sexual acts or were struggling to cope.

"He would say the word 'Thank you, sir' when he had reached a point when he was reaching his limits and I should start to pay a bit more attention to what I was doing," Brizzi said.

But the pair also had the code word "red" to indicate that the activity should stop immediately.

"He never said red," Brizzi told the court.

The Italian denies murdering the officer but has admitted obstructing a coroner by disposing of the body.

Pathologist Dr Ashley Fegan-Earl told the court it was "possible" that Pc Semple had died from a heart attack, rather than strangulation.

Ms Bennett-Jenkins asked the pathologist whether Pc Semple's history of having high cholesterol and being overweight could have caused him to suffer a cardiac arrest.

"Of course the heart was not present to be examined but based on our knowledge of heart disease ... yes, I think that it is possible."

He added that it was a "plausible" cause of death but stressed that "no-one was able to examine the heart".

The pathologist for the prosecution gave evidence earlier this week and said that, in the absence of the internal organs, he had recorded the cause of death as "unascertained" because Pc Semple's body had been dismembered and parts were never recovered.

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

The trial previously heard that Brizzi was trying to dissolve Pc Semple's dismembered body parts in an acid bath when police called round to his home to investigate complaints of a putrid smell.

Brizzi was arrested on April 7 and told police he thought he was "getting away with it" when he cut up and disposed of the body, the court has been told.