Hamburg - In a case alleged by prosecutors to have been an "honour killing," an Afghan-born man was sentenced to life in prison on Friday for stabbing to death his own sister.

Morsal O, aged 16, was knifed to death on May 15, 2008, a day after she approached welfare officials in the northern German city Hamburg for protection from her brother, Ahmad-Sobair O, now 24.

He had objected to the pretty schoolgirl wearing fancy clothes and staying out all night.

He was convicted Friday of murder.

The case gained Germany-wide attention, amid a controversy over whether Muslim immigrants are more violent than native Germans.

Critics accuse immigrants of importing the archaic central Asian custom of honour killing. The term refers to a Muslim daughter being executed by the males of her own family, who jointly decide she has disgraced family honour through a sexual relationship.

That interpretation came under attack at the trial, as the ethnic Tajik family had not been devout. The father had been a fighter pilot in the communist air force in Afghanistan and fled when the Taliban took over the country.

The defence rejected the honour-killing explanation, contending that the immigrant family had been simply dysfunctional. O admitted the stabbing after his arrest, but declined to testify in court.

A court-appointed psychologist said Ahmad-Sobair O had a personality disorder that made his temper uncontrollable.

Prosecutors, who had sought an exemplary sentence to deter future honour killings, asked in vain for the psychologist to be dismissed and a mistrial declared.

Defence lawyers welcomed the psychologist's testimony. They said O killed his sister in a fit of sudden passion and did not deserve the most severe penalty. Defence lawyer Thomas Bliwier denounced the "politicking" in Germany over the case.

Witnesses said Morsal and her brother had a relationship that was a mix of love and hatred, with the spirited, fun-loving girl refusing to bow to her violent, domineering, older brother.

Morsal, who had won a civic prize for setting up a peer-counselling project at her school, had complained several times to youth-aid counsellors about her brother and parents.

Morsal and her elder brother were both troubled teenagers. In the big city of Hamburg, the parents had difficulty managing them.

The brother killed Morsal after seeing her walking with boys and being told by his own friends that Morsal would become a prostitute.

Just before the trial ended, the defendant, who had several previous convictions for assault, suddenly burst into tears and sobbed that he had never meant to kill Morsal.

"She was my own sister," he shouted.

German campaigners against honour killings demonstrated outside the courtroom. Heidemarie Grobe of the group Terres des Femmes insisted that Morsal's death fitted the honour-killing pattern.

"It's fixed in the roles inside the families," she said. "The brother usually claims the monopoly of force over the sister."

Terre des Femmes estimates 50 women have been killed in the past decade in Germany for reasons of supposed honour. (dpa)