The girl’s father, Chomir Ali, 41, who bullied his two young British-born sons into killing Arash Ghorbani-Zarin, 19, boasted that the murder was to "vindicate the family’s honour" after the popular engineering student made Ali’s daughter pregnant.

With so-called honour killings on the increase in Britain, detectives described this as one of the most tragic cases they had encountered.

Friends of the lovers had warned them not to be seen holding hands in public but Mr Ghorbani-Zarin insisted that no number of threats would make him give up his girlfriend. Gentle-natured and a Muslim himself, he said that it was not his intention to insult her family but felt that he had nothing to hide or apologise for.

Manna Begum, 19, defied her father’s demand that she marry a man she had never met in Bangladesh and tried to kill herself after her family held her prisoner at home. Her father even tried to stop paramedics getting into the house to take his daughter to hospital after she slashed her wrists.

Days later, in October last year, she ran away after secretly getting engaged, but was forced to return to the family home.

Her boyfriend had taken a part-time job to support them and was on his way to collect Manna in his car when he was ambushed by her two brothers. The older brother, Muji Rahman, 19, trapped his victim with his seatbelt before stabbing him so violently in the chest that the tip of the blade broke.

The jury at Oxford Crown Court was told how Muji Rahman was a swaggering bully who, while condemning his sister’s behaviour for flouting the family’s strict Muslim code, had had sex before marriage, drank alcohol and rarely bothered to go to the mosque. After the killing in November last year he went out clubbing with friends in Oxford city centre.

His younger brother, Mamnoor, who was 15 at the time, described how he had often acted as a secret go-between for the lovers but was beaten and brainwashed by his bad-tempered father to take part in the murder.

Chomir Ali, who was out delivering takeaway meals while his sons carried out the murder, was arrested after his botched attempt to get rid of the knife and the killers’ bloodstained clothes.

Yesterday all three stared straight ahead, showing no emotion as the jury returned a unanimous guilty verdict. In the public gallery the family of the Iranian-born Mr Ghorbani-Zarin shouted at his killers and embraced detectives.

Police described how, after the murder, Miss Begum was forced into terminating her pregnancy.

Last night in a statement read by Detective Chief Inspector Steve Tolmie of Thames Valley Police, the parents of Mr Ghorbani-Zarin said: "This verdict gives us back some comfort that justice has been done but it does not bring him back."

During the case the victim’s father, Raheem Ghorbani-Zarin, 49, a civil engineer, condemned the way of thinking that led to his son’s murder. "I’m a Muslim too," he said. "But this is not something that Muslims support."

He said that, although obedience to one’s father in the Muslim community was important, there were limits to what children should obey. Mr Justice Gross warned all three that they faced life imprisonment but he adjourned setting the minimum terms.

Schoolfriends had introduced Miss Begum to the soft-spoken Mr Ghorbani-Zarin, who was studying at Oxford Brookes University. Rather than wear a Muslim headdress, she preferred to dress in jeans and hooded tops. When her father learnt of her relationship he stopped her from going to school or talking to anyone outside the family.

When her older brother, Muji, saw her with her boyfriend in a local shop he attacked her, hitting her three times in the face and threatening her partner. One of their friends, James Ware, said: "Arash loved Manna unconditionally. They both loved each other. How can that be so wrong?"

Jomsed Ali, 60, a member of Oxford’s Bangladeshi community, said that the actions of Chomir Ali and his sons had been condemned. He said: "What he did is very bad, for his family and his community. This is the first time I have ever seen this."

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