LONDON — British authorities have convicted more than a dozen sex offenders in a northern English city, but children’s advocates expressed outrage that police paid a convicted rapist to infiltrate specially organized parties where young women and girls as young as 15 were plied with drugs and alcohol before being abused.

The revelations came after 17 men and one woman were convicted of or admitted to charges including rape, supplying drugs and inciting prostitution following a series of trials at Newcastle Crown Court. Reporting restrictions imposed by the court prevented details of the cases from emerging until now.

Chief Constable Steve Ashman of Northumbria Police described exploitation as the challenge of this generation and defended the decision to hire the informant, known only as XY. Because of the evidence provided by XY, those convicted in this case will spend lengthy terms in prison, Ashman said, adding that strong punishments were needed to deal with “vile” individuals who target vulnerable people.

“He was a convicted rapist and to some of us the thought of the police engaging with such a person and paying them for information may appear repugnant,” he said. “However, he proved he was in a position whereby he could, and did, alert police to situations which allowed them to prevent offending and provide safeguarding measures towards potential victims.”

The use of such informants is always overseen by a senior police officer and is subject to independent review, he said.

But child rights advocates said that regardless of police officers’ good intentions, their “misguided actions” violated accepted child protection procedures and could have jeopardized the investigation of these abusers. The victims in this case ranged in age from 15 to their early 20s, according to prosecutors.

“We are appalled to learn that police paid a child rapist and planted him in the midst of vulnerable young girls,” said Jon Brown of the National Society for the Prevention of Cruelty to Children. “You just couldn’t make it up.”

The informant’s role was revealed when defense attorneys challenged some prosecution evidence. XY told the court he had to go along with what was happening so he could continue to gather evidence.

“I was chilling with the boys,” he said during an abuse of process hearing. “I had to make it look like I was their friend.”