The former police chief who set up Operation Sanctuary said she was "kept in the dark" about the use of a convicted child rapist as an informant.
Sue Sim, who retired in 2015, said she was appalled and would never have agreed to his use or the payment.
Information provided by the man, known as XY, led to the conviction of 17 men and one woman for abusing girls in Newcastle, Northumbria police claimed
The force said it was inappropriate to comment on individuals' opinions.
It emerged during the court processes that XY was paid £10,300.
The current chief constable, Steve Ashman, has defended the move, saying he was "certain as I can be" that it was "morally" right.
"What we've ended up doing here is putting dangerous men behind bars and protecting vulnerable women and girls that we simply wouldn't have been able to do if we didn't have that jump off point we got [from the information]," he said.
The use of XY was "inside the law and inside accepted policy and practice", he added.
However, Ms Sim, who following her retirement made allegations about sexism within the force, has called for a public inquiry.
She said: "I created this operation and I'm delighted at the success of it, in that 18 people who should be off the streets, are now off the streets.
"But I'm very, very surprised at this as a tactic. I'm absolutely horrified, appalled.
"I would never have agreed to the payment of a convicted child rapist.
"I keep hearing it was a very difficult decision - clearly it was so difficult that they didn't tell me. Probably because they knew I would totally and utterly disagree."
There were a number of overt and covert tactics which could have been put in place, she said.
"The big issue with this one is, I haven't read anything, I haven't heard anything in the news, that actually says what evidence was actually produced by this informant?
"Two wrongs don't make a right - he was a convicted child rapist and you can never put sufficient safeguards in place."
She added: "The success of the operation should not be tarnished by this, but it does need further serious investigation.
"I feel so passionately about this that I would like to apologise to any rape victims who feel they have been let down by this."
Northumbria Police said in a statement it had been "open and transparent" in its response to Operation Sanctuary and the use of XY, but it "wouldn't be appropriate" to comment on individuals' opinions.