A Pakistani man demanding justice after his pregnant wife was murdered outside Lahore's high court this week admitted on Thursday to strangling his first wife, in an admission that is likely to focus even more attention on the prevalence of so-called "honour" killings in the country.

Muhummad Iqbal, the 45-year-old husband of Farzana Parveen, who was beaten to death by 20 male relatives on Tuesday, said he strangled his first wife in order to marry Parveen.

He avoided a prison sentence after his family used Islamic provisions of Pakistan's legal system to forgive him, precisely those he has insisted should not be available to his wife's killers.

"I was in love with Farzana and killed my first wife because of this love," he told Agence France-Presse.

Police confirmed that the killing had happened six years ago and that he was released after a "compromise" with his family.

Iqbal has also claimed that Parveen's family killed another one of their daughters some years ago. Speaking to a researcher from the Aurat Foundation, a women's rights organisation, he claimed that Parveen's father, Muhammad Azeem, had poisoned the other woman after falling out with her husband-in-law.

The foundation has been unable to confirm Iqbal's claim about a second killing.

The extraordinary twists to the affair came after Pakistan's prime minister, Nawaz Sharif, ordered an urgent investigation into the killing of Parveen, a woman who had enraged her family after marrying without their consent.

In a statement he said the crime was "totally unacceptable and must be dealt with in accordance with the law promptly".

He also ordered the chief minister of Punjab province, his younger brother Shahbaz Sharif, to take immediate action and launch an urgent investigation.

The deadly attack on Parveen, which reportedly lasted for around 15 minutes, began soon after she and Iqbal arrived at the court where she was due to testify against her father's claim that she had been kidnapped and coerced into marriage.

Her father, who is the only one of the group to be have been arrested so far, told police that his daughter had been killed because he had dishonoured her family.

Iqbal has claimed that Parveen's father only withdrew his support for their marriage after demanding more money than had initially been agreed at the start of a long engagement. Sharif's intervention followed international uproar, including a lengthy and stinging condemnation from the UN high commissioner for human rights, Navi Pillay, who said Pakistan must take "urgent and strong measures to put an end to the continuous stream of so-called 'honour killings' and other forms of violence against women".

She said: "The fact that she was killed on her way to court shows a serious failure by the state to provide security for someone who – given how common such killings are in Pakistan – was obviously at risk."

The Human Rights Commission of Pakistan said that the media had reported that nearly 900 women had been killed in "honour" crimes in 2013 alone, but the actual figure is likely to be far higher.

Until Thursday there had been little comment on the case domestically, with newspapers and television stations focussing on other stories.

One journalist, an editor of an Urdu national paper who did not want to be named, said the country's media reflected its audience.

"Although we have some educated people, most are still living in semi-tribal societies in far-flung rural areas," he said. "In a country where people are being killed every day by miscreants and militants it is not so important when one woman is killed by one husband."

Some members of the public in Lahore clearly share the media's ambivalence.

Muhammad Yaqub, a student at a private university in the city, said he understood the loss of honour for the family but disliked the brutal way the woman had been killed.

"He did some right and some wrong," he said.