When Seeta Kaur arrived in India with her children for a three-week visit to her husband’s relatives she was determined that when she returned to Britain it would be with both the sons who had accompanied her.

Since giving birth to the boys, who were aged 10 and two when they travelled to India, Seeta had confided in her family and close friends about the domestic abuse she suffered at the hands of her husband, Pawan, for resisting his demands to give one of them away.

Pawan, an Indian national, lived with Seeta and their four children (they also had two daughters) in Edmonton, north London, where she was born and raised. He wanted his brother and sister-in-law in India to have one of their sons because they could not have children of their own and, at the end of the trip, threatened to leave one of them behind.

Informal inter-family adoptions are not uncommon in India, because of an inefficient and disorganised legal adoption system. According to the government’s own figures, 3,000 children were adopted formally in the country last year, while there are an estimated 30 million orphans in the country.

She was under pressure to give one of her sons away. She was a British woman and the idea horrified her Swinder Singh, sister of Seeta Kaur

Swinder Singh, Seeta’s sister, said: “She was open with us about the volatile relationship with her husband and the huge pressure she was under to give one of her sons away, which he had made a matter of family honour. She was a British woman and the idea horrified her.”

On 31 March last year, Pawan phoned Seeta’s family in north London to tell them she had died of a heart attack at her in-laws’ home in Kurukshetra in the north Indian state of Haryana.

They flew to India and began to suspect she was the victim of an “honour” killing when they saw bruising around Seeta’s neck and upper chest as she lay in her coffin. They announced their plan to take her back to Britain the following morning for a postmortem … but awoke to discover that Seeta had been cremated while they slept. When the case was closed by Indian police without a full investigation, Seeta’s family turned to the Metropolitan police for help, but say they were initially told that British officers had no jurisdiction to investigate because the alleged crime took place abroad.

The Foreign and Commonwealth Office said it could not get involved, or help secure the return of Seeta’s four children, who remain in India with their father, despite the high court ordering their return in April last year.

The family’s fight is now being led by the women’s group Southall Black Sisters, who have amassed a dossier of evidence and launched a “Justice for Seeta” campaign. They are backed by Kate Osamor, the family’s local MP, and Naz Shah, who has led the fight for justice for Samia Shahid, the Bradford woman killed in Pakistan last July.

They are calling for the Metropolitan police to look into Seeta’s case, and demanding new legislation that would make it easier to investigate “honour” killings of British citizens abroad, claiming a legal grey area is letting perpetrators evade justice. Pragna Patel, director of Southall Black Sisters, said: “Men are taking women abroad and getting away with honour crimes because the law is not clear on what exactly is the duty of British authorities to investigate them.”

Describing the current situation as a “lottery”, she cited cases such as that of Madeleine McCann and missing toddler Ben Needham as proof that British police are willing to investigate crimes abroad in some instances. “Clearly, when there are non-white Britons involved and it’s an ‘honour’ crime, then there appears to be a dereliction of duty on the part of the British police. This is what has happened in Seeta’s case, because we have collected concrete evidence and yet the Metropolitan police have not acted upon it.”

This evidence includes 26 witness statements from friends and relatives with details of the abuse they say she suffered for opposing her husband. One of her closest friends testified that, ahead of the trip to India, Seeta confided to her that she feared for her life. It was also said that the couple had a heated argument on the night she died. Medical reports showed that Seeta, aged 33, had no condition to explain a heart attack.

Shamik Dutta, solicitor for Seeta’s family, said: “A new law which recognises the international nature of honour killing would help families secure justice and act as a deterrent against such offences.”

The dossier compiled by campaigners was sent to the Metropolitan police in August, but no response has yet been received. The police said in a statement: “We are in the process of responding to a number of queries raised with us by a firm of solicitors acting on behalf of the family of Seeta.”