A chief constable said dangerous men would not be behind bars if he had not decided to pay a convicted child rapist almost £10,000 to spy on parties where it was suspected under-age girls were fed drugs and sexually abused.
The NSPCC was "appalled" Northumbria Police chief Steve Ashman authorised the paedophile's deployment, which can only be reported now that 18 people have been convicted or admitted offences prosecuted in a series of trials related to child sexual exploitation in Newcastle.
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.
Mr Ashman, who is due to retire, accepted some people will find his decision to use XY "very, very difficult to accept".
Defending the deployment, he 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."
He added: "We have to step into a murky, a dangerous and a shadowy world and the people who are going to provide us with that information that will protect victims, that will stop other women and girls becoming victims of this abuse, it's not the postmaster or the district nurse, or some other person in a position of authority.
"They are the very people who themselves may well have committed these vile acts.
"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 deployed to attend parties, although he could not be 100% sure the informant stuck to those rules.
He said: "I'm a little concerned that people have got the impression in their heads that we were sending him into these sessions – we weren't.
"This is about finding out who is going, where they are taking place, what car is such and such driving, where is he living at this moment in time, does he have access to drugs, where do they buy the drugs from.
"It's not about someone being amongst the offending."
XY has told a court he did attend "one or two parties" but left before any offending happened.
Jon Brown, of the NSPCC, 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."
The force's police and crime commissioner Vera Baird said it was a difficult decision to use XY.
She said: "I would have wished this man not to be used, in particular because of his conviction for rape.
"But, I have questioned the chief constable and, in liaison with other senior officers, Mr Ashman has satisfied me that the difficult moral decision to use the informant was taken with care and with particular regard to the welfare of victims."
She was assured the evidence could not be have been obtained in any other way and that his information led to "the speedier rescue and safeguarding of vulnerable women".
It has also emerged a police officer was fired for mishandling evidence which could have stopped one abuser two years after he was first arrested.
The startling information about XY came out during pre-trial hearings in Newcastle which attempted, but failed, to halt prosecutions against a number of men accused of a range of serious offences including drug dealing and sexually abusing girls.
During the proceedings in October and November, barristers argued over whether the cases of more than 10 men should be thrown out.
It was argued the public's confidence in the justice system would be "diminished" if the trials went ahead, given the rapist XY had acted as an informant, formally known as a covert human intelligence source, or "CHIS".
Robin Patton, representing one of the defendants, said XY was paid £9,680 over 21 months by Northumbria Police for informing.
Mr Patton said XY was a "convicted child rapist who drugged a child and invited someone else to rape her after he had" and was subject to a suspended sentence when he was deployed by police in 2014.
Mr Patton said police claimed they carried out a risk assessment, but that the "very next day" after he was recruited, XY was in court for a dishonesty offence.
In September 2015 XY was arrested on suspicion of inciting sexual activity with a child after a teenage girl claimed a man approached her and made an indecent proposition.
The informant was later told he would face no action after he took part in an identity parade.
Mr Patton said: "I have tried to think of convictions that make him less suitable to act as a CHIS in an operation of this sort... I have not been able to."
David Hislop QC, representing another defendant, said XY had 13 previous convictions, including 26 offences of dishonesty.
During the legal submissions, XY gave evidence to the court and made a series of lurid allegations against the police, including claims of racism and that he was asked to plant drugs.
Judge Penny Moreland rejected his evidence in its entirety, describing it as "inherently unreliable" and "clearly dishonest".
XY claimed he was recruited because he acted as an informal taxi driver for some of the defendants.
"I would get to know where they pick up their drugs, where the parties were," he said.
At another point, he claimed: "I was chilling with the boys. I had to make it look like I was their friend."
He told the abuse of process hearing that police tasked him "to find out what was going on in the area, when parties were taking place, where there was criminal activity".
"There were certain individuals they were very interested in, which I was close to," he said.
He added: "When I worked for (Operation) Sanctuary I actually believed I was doing good.
"I never, ever thought it was bad. I enjoyed it."
As well as receiving money, police informants were given "texts", secret letters that could be put before a judge if they were convicted of an offence to gain a more lenient sentence.
Judge Moreland turned down the abuse of process application, ordering the trials of the defendants should not be thrown out.
She also found there was no evidence that XY was guilty of any sexual misconduct towards any complainants in the cases.
In a further statement, Mr Ashman said police told XY to stay away from parties.
He said: "XY was not paid to attend or arrange these sessions.
"On the contrary XY was directed not to go to sessions and was told that if he did he would be liable for arrest."