Banaz Mahmod, 20, was strangled with a bootlace and her body buried in a suitcase in a back garden last January because her Iraqi Kurdish family disapproved of her Iranian Kurdish boyfriend, the Old Bailey heard.

Mahmod Mahmod, 52, and his brother, Ari Agha Mahmod, 51, of Mitcham, south London, were found guilty after a three-month trial at the London court.

Darbaz Maref-Rasull, 24, of Hounslow, west London, was cleared, with Ari Mahmod, of conspiracy to pervert the course of justice. Pshtewan Hama, 26, also of Hounslow, had already pleaded guilty on the same count.

The jury was told that Mohamad Hama, 30, of West Norwood, south London, an associate of Ari Mahmod, had already pleaded guilty to the murder. He killed Ms Mahmod at the instigation of her family, Victor Temple, prosecuting, said.

Ms Mahmod told police at least four times that she feared they wanted to kill her, even writing a letter naming those who she thought would do it - one of whom later admitted his part in the killing and two who fled the country.

On another occasion her fears were dismissed by a female police officer who thought she had made up the story to get the attention of her boyfriend, Rahmat Sulemani.

Police today admitted they needed to "take stock" of the lessons to be learned during the investigation.

During the proceedings, the jury watched a mobile phone recording of Ms Mahmod, made by Mr Sulemani after an earlier attempt on Ms Mahmod's life.

In the recording she claimed her father forced her to drink brandy to make her sleepy. "It was in a black plastic bag like in Kurdistan," she said. "He said, 'Open it up and drink it bit by bit'."

She became frightened after her father came back into the room wearing gloves. The curtains were drawn and cartoons were playing on the television.

"I was really scared and got up to leave the room. He said, 'sit down, sit down'."

When her father left again she escaped by a back door and tried to raise the alarm by smashing neighbours' windows.

The video was filmed by Mr Suleimani on a hospital trolley, at St George's in Tooting, on New Year's Eve 2005, and was played in court on video screens.

Ms Mahmod fled but later went back to her family and tried to carry on her relationship with Mr Sulemani in secret.

But when they were discovered and Mr Sulemani was threatened by Ari Mahmod's associates, she contacted police again.

She was urged to stay at a safe house but told officers she believed she would be safe at home because her mother was there. The following day, on January 24 2006, she disappeared. Her body was discovered in Handsworth, Birmingham, three months later.

A number of police officers face an internal disciplinary investigation over the handling of the case.

Commander Simon Bray, in overall charge of the murder inquiry, called Operation Braidland, admitted honour-based crime was difficult for police to "identify and investigate".

"Following Banaz's murder it is necessary for us to take stock on the progress we have made in tackling HBV [honour-based violence] and to continue to identify ways in which we can improve our response," he said.

"Banaz contacted the MPS [Metropolitan police service] on a number of occasions and we did take action in all of those cases. Whether those actions were appropriate will be a matter for the directorate of professional standards review."

During the inquiry, 47 premises were searched, 22 people arrested, 16 vehicles seized, 779 statements taken and 1253 documents created, police said. A total of 2254 exhibits were collected and 26 interviews conducted, amounting to 113 tapes.

Mr Sulemani said Ms Mahmod meant the world to him.

"I don't think there are many couples who could say that their love is stronger than mine and Banaz's," he said. "I know it is too late for me and Banaz. If there's anyone out there in the same situation, do something about it before it's too late. Once it's too late, it's too late - you will never get your life back."

Ms Mahmod's family came to Britain as asylum seekers when she was 10. She entered an arranged marriage at the age of 17 with her family's encouragement. But her husband beat her and the marriage failed, so she returned to live at home.