A man accused of stabbing a female special constable confessed to murder in a bar, an Old Bailey jury was told.
Mrs Patel-Nasri was stabbed outside her home
Jason Jones is alleged to have murdered Nisha Patel-Nasri, 29, at her home, on the orders of her husband Fadi Nasri.
Witness Daniel Fox said Mr Jones told him he went to the house to intimidate her, but she had a knife. He claimed Mr Jones told him: "It was me or her."
Mr Jones, Mr Nasri and two others deny murdering Mrs Patel-Nasri at her home in Wembley, north London, in May 2006.
Mr Fox told the jury: "[Mr Jones] said he went into the house and the police officer, she had a knife in her hand. She had a knife and he didn't."
"He said 'when someone's in front of you with a knife, it was me or her' and he stabbed her in the leg. He said she'd died because he'd hit the main artery."
'Clear my conscience'
Mr Fox, who admitted he had been in trouble with the police before, said he told officers about the conversation with Mr Jones, when he was being held at a station on another matter, because he wanted to "clear his conscience".
He said he first met Mr Jones about two years ago and had the alleged conversation with him at Soma Bar in Stepney Green, east London.
It took place not long after Mr Jones's arrest in December 2006 over Mrs Patel-Nasri's murder the previous May, he said.
Mr Nasri, 34, a limousine hire boss, of Alderman Court, Barnet, north London, Mr Jones, 36, of Hathaway Crescent, Manor Park, east London, and Tony Emmanuel, 42, a driver, of Clements Road, East Ham, east London, each face a charge of murder.
Also accused is drug dealer Rodger Leslie, 38, of Chesterfield Flats, Bells Hill, Barnet. All four men deny the charge.
The trial was adjourned until Tuesday.