Paying £10,000 to a convicted child rapist to infiltrate a paedophile gang "crossed a line", it has been claimed.
Jim Gamble, who set up the Government's taskforce to fight child sexual exploitation, criticised the move by Northumbria Police.
The force's chief constable had defended the decision, saying dangerous men would not be behind bars if he had not decided to pay the offender to spy on parties where it was suspected under-age girls were fed drugs and sexually abused.
But Mr Gamble, former chief executive of the Child Exploitation and Online Protection Centre, said the tactic went "way over the line".
He told the BBC: "I think all police forces are under pressure to get things right.
"But there are ways and means to doing it ... there need to be limits and there should be lines that shouldn't be crossed.
"In my opinion they have gone way over the line on this one.
"Personally, I can't envisage any circumstances where I would have authorised payment to someone convicted of rape.
"I can't imagine how you could have control mechanisms in place with an informant of that type ... that would give you reassurance that they didn't still represent a risk to young and vulnerable women, given what I know about this person's history."
The informant, known only as XY, was recruited despite being a sex offender who had drugged an under-age girl and invited another man to rape her after he had done so, Newcastle Crown Court heard.
Years later, the force recruited him to work as an informant on the massive Operation Sanctuary inquiry, one strand of which, known as Operation Shelter, has just finished going through the courts.
Defending the deployment, chief constable Steve Ashman told a news conference: "It's a decision that we've had to wrestle with ourselves but I can categorically state sitting here today that there are dangerous men behind bars now and vulnerable people protected that would not have been the case had we not used that informant.
"This is the world that we have to step into in policing and it is dangerous and it is difficult but that is what we are prepared to do.
"We'll do everything we can within the law to bring these people to justice."
Mr Ashman insisted the parameters stated XY was not to be sent to attend parties, although he could not be 100% sure the informant stuck to those rules.
Jon Brown of the NSPCC said the charity was "appalled" by the revelations.
He said: "We are appalled to learn that police paid a child rapist and planted him in the midst of vulnerable young girls.
"You just couldn't make it up.
"It beggars belief that it would ever have been considered, let alone approved, and serious questions must be asked about the force's approach to child sexual exploitation operations."
Speaking to Channel 4 News, Rotherham sex abuse survivor Sammy Woodhouse said: "He (the informant) has been placed back in their with vulnerable people who could have been hurt.
"It's great to see these people behind bars, but I think there could have been better ways of doing that."
A total of 17 men and one woman have been convicted of, or have admitted, charges including rape, supplying drugs and inciting prostitution, in a series of trials at Newcastle Crown Court.
Older men preyed on immature teenagers who were plied with cocaine, cannabis, alcohol or mephedrone, then raped or persuaded into having sexual activity with the lure of the illegal drugs at parties known as "sessions".
The wider operation has seen around 100 people convicted of a range of serious offences, including drugs, modern-day slavery and firearms charges, with jail terms totalling more than 300 years.