A 15-year-old boy told a court how he stabbed a suspected paedophile because he was trying to abuse his girlfreind's 14-year-old sister.

The boy walked into Robert Daley's kitchen and saw him with one hand around the girl's throat.

The boy said: "He [Daley] had his left hand on her throat and his right hand was trying to touch her."

The boy described how he tried to pull Mr Daley, 45, off the girl, but Mr Daley hit him in the side of the face, causing the boy to fall backwards.

The boy, who was 14 at the time of the offence, said to Daley "what are you doing, please stop", the court heard.

He said: "I tried to grab [the girl] and that is when Daley reached for a knife.

"I looked around to see what I could use as a weapon. He was getting a knife and I just got scared after all the stories he had told me of things that he had done."

The boy told the court he found a two- to three-inch knife in the kitchen sink and "poked" Mr Daley with it.

He said when Mr Daley did not let go of the girl he poked him again.

The court heard the boy later told police it was a "gentle poke with no real force".

After the incident the boy and the girl ran from the flat.

The boy, from Clapham, and the 14-year-old girl, now aged 15, both deny murder.

Mr Daley was stabbed five times in his kitchen just an hour after discovering he would not face prosecution for a string of alleged sexual assaults, the Old Bailey heard.

Two blows from the knife penetrated Daley’s heart, causing massive blood loss and subsequent death.

Police had investigated claims of sex attacks against her 16-year-old sister and another woman but had dropped the case because of a lack of evidence, the court heard.

The Streatham girl also claimed Mr Daley sexually abused her from the age of 12, but prosecutors never looked at the case because she had not made a formal statement, the court was told.

The two alleged killers went to Robert Daley’s flat in Ferndale Road, Brixton, shortly before 6pm on April 23 this year, after the elder sister telephoned them to say the police had dropped the case.

The case continues.