A teenager was cleared yesterday of the murder of a 17-year-old whom she stabbed through the heart during a violent clash between female gangs.

Chelsea Bennett, 19, admitted plunging a steak knife into the heart of Sian Simpson during a row over a man. The argument centred on Nathan Davidson, whose pregnant girlfriend, Daniele Cooke, was Miss Bennett's best friend. He had a four-month-old son with Miss Simpson's cousin, Chantelle Campbell.

Police found between 12 and 30 girls and young women when they arrived at the scene of the fight in Croydon, South London, in June last year, the Old Bailey was told.

The fatal stabbing, resulting from fighting between groups of girls, may have been the first of its kind, a senior detective said after the trial. He said the killing was another example of a trivial row leading to death because of the presence of a knife.

Brian Altman, QC, for the prosecution, had told the court that Miss Simpson's death was the culmination of an afternoon and evening of tension between two groups of young females. “Resort was made, all too depressingly and familiarly, to the use of a knife to settle the score, with a fatal outcome,” he said.

Miss Simpson, an A-level student who dreamt of becoming an accountant, was left lying in a pool of blood, surrounded by young girls screaming at each other.

The court heard that the fight began when Miss Cooke decided to confront Mr Davidson, 26, when she heard that he was with his former partner and their child at his mother's house. She left a voice message on his mobile phone saying: “You hear what I am saying, yeah? I am coming to your mum's house so if she's there she's going to get f***** up and so are you.”

When Miss Cooke and Miss Bennett arrived at the address with three other women there were threats between the rival groups, “doubtless borne of jealousy”, Mr Altman said.

Miss Simpson and Miss Campbell, who had armed herself with a kitchen knife, went to confront the girls and several scuffles broke out. Miss Bennett said in evidence that Miss Simpson - who jurors heard was nicknamed “Pitbull” - had come at her with a knife first, so she grabbed a blade to defend herself. “She had the knife and she kept coming. She jumped on my back and started punching my head,” Miss Bennett told the jury. “I saw the knife on the pavement. I picked it up. I thought she would be scared. I thought she would stop running towards me. I wanted to get back into the car and her to stop running at me. I got up very quickly. I was trying to block her and push her away.”

After the stabbing, Miss Bennett locked herself in Miss Cooke's car before the vehicle was mobbed, with girls kicking out and hurling bricks at her. Police arrested her at the scene, when she pulled the brown-handled steak knife, used to stab Miss Simpson, from under the car seat.

Miss Bennett told the court: “I wanted to write a letter to Sian's family. I wanted to say maybe they can understand how scared I was and why I picked up that knife and I didn't even know that I had done it.”

She sat shaking in the dock as the verdicts were delivered and sobbed uncontrollably as the jury cleared of her murder and manslaughter. Miss Simpson's family shouted abuse.

The two rival groups of women have continued their feud and a tribute website has been taken over, with girls on both sides abusing one another. Miss Bennett's friends have written messages insulting Miss Campbell and blaming her and Mr Davidson for the death. Miss Simpson's friends and relatives have threatened Miss Bennett. Under one message, “halt the fussing and respect the dead”, a friend of Miss Simpson's pleaded: “This site is for condolences ... not tit 4 tat about who did what with who. Please allow the poor girl to rest in peace ... have you no shame? It is apparent this ‘dispute' will cease in another loss of life. Because u so-called pals seem bloodthirsty!”

Miss Simpson's aunt, Sharon Fairclough, spoke out against knife crime in a statement outside court, flanked by Sian's mother, Mikaala Fairclough, and the girl's grandmother and other relatives. She said: “Our baby Sian Simpson was a good girl, a believer in what's right. She had a right to life. It is time parents, guardians and responsible adults took back our roles as disciplinarians. We, as the bearers and carers of these children, must instil morals in the youth of today. Sian Simpson, loving sister, daughter, granddaughter and niece, was a good, well-mannered humble young woman.”

She said Miss Simpson had received an acceptance letter from a university shortly after she died. “She was a good girl doing a good deed,” Ms Fairclough added. “She tried to stop the violence. She was there to protect, she was not in a girl gang.”

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