A teenage girl who was being sexually exploited was enlisted by police to collect information about her pimp, which led to her becoming an accessory to murder, a member of the House of Lords has revealed.

In a Lords debate on the use of child spies, peers said 14 minors had been recruited since 2015. Sally Hamwee revealed the case of the 17-year-old, who was “on the edge of care”, and was one of a group of girls the suspect – whom “she thought of as her boyfriend” – had been selling for sex.

“The police were looking for information on him and she was left in her situation so that she could provide information. In other words, she was exploited by him and continued to be exploited by him, and was, arguably, exploited by the police,” Lady Hamwee said.

“Eventually, she witnessed a murder. She was drawn into it, and not just as a witness, as she was asked to dispose of clothes and other items afterwards.”

Hamwee said she had been told about the girl’s case by a contact within Safer London, a charity that works with young people caught up in gang violence and sexual exploitation. She told the Guardian she knew no more about it than she had told the house.

The Lib Dem peer spoke last week at an evening debate on a motion of regret over the government’s use of juvenile covert human intelligence sources (CHIS), revealed by the Guardian in July when the government sought to increase these powers through secondary legislation.

After condemnation from politicians and human rights groups, the matter has become the subject of an investigation by parliament’s joint committee on human rights. It has criticised the government over an apparent lack of safeguards in the system and questioned whether it can be compatible with international law.

The case cited by Hamwee is markedly different from those referred to by the government in its attempts to justify the use of child spies. The Home Office has twice directed questioners to a study by Dr Brian Chappell, a former police officer turned lecturer in criminal investigation at the University of Portsmouth, whose research “illustrated … positive life course outcomes for individuals following their role as an authorised juvenile CHIS”.

Preliminary findings of the scale of the use of child spies were also revealed for the first time during the debate. Susan Williams, a junior Home Office minister, said the Investigatory Powers Commissioner’s Office found 14 children had been authorised as CHIS by police forces between January 2015 and October 2018.

Lady Williams said she hoped it “gives noble lords some comfort” to learn that most were aged 17, and that none of the UK’s 43 police forces had authorised a child spy who was younger than 15. It is unclear whether the figures include data from the security and intelligence services or other agencies with the power to use child spies.

“In the majority of instances, the types of criminality on which they have provided information in the past three years are markedly serious and include murder, gang violence, high-level drug supply and the possession or use of offensive weapons,” Lady Williams said.

A campaign to crowdfund a legal challenge to the government’s use of child spies has achieved two-thirds of its minimum target with two days to go. Just for Kids Law is trying to raise £6,000 to fund a judicial review of the power. However, it faces massive potential costs if the challenge fails after the Home Office refused a request for a cost neutrality agreement – leaving it on the hook for the cost of government lawyers.

Enver Solomon, the legal charity’s chief executive, said the case raised in the Lords by Hamwee confirmed its worst fears about the way investigatory agencies use child spies.

“It seems that the police asked the child to prolong a traumatising and dangerous situation when they could have rescued her from it,” Solomon said. “It shows just how important it is that we challenge the government over what can be a damaging and exploitative practice.”