A sadomasochist accused of murdering Greenhithe policeman Gordon Semple has told a court the officer died while they were playing "a very dangerous game".

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

The pair, who met on the gay dating app Grindr, had been performing bondage actions - which included Brizzi sitting on Mr Semple's face while he rocked his "bottom back and forward", the jury at the Old Bailey heard.

Brizzi said Mr 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."

He said Mr Semple was "extremely excited … while we were playing this game and was enjoying it".

"His face was under my buttocks," Brizzi said, but when he stood up he noticed "that something had gone wrong because he didn't respond any more".

"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 Mr Semple's mask and "realised that he was completely unresponsive and I started to panic".

He slapped Mr 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 Mr Semple may have a "condition" such as asthma.

He said he checked Mr 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 he did not call an ambulance, he replied: "Yes, I accept that."

Brizzi also told the court he and Mr 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 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.

The trial continues.