A California privacy group is concerned about a Los Angeles City Hall proposal to send warning letters to the owners of cars seen driving in areas known for prostitution.

The City Council voted Wednesday to ask the city attorney’s office to weigh in on sending “john letters” to the registered owners of vehicles spotted on streets that draw prostitutes.

The motion was put forth by San Fernando Valley Councilwoman Nury Martinez, who has led efforts to fight prostitution in her district.

But the plan drew alarm from Dave Maass, an investigative researcher with the San Francisco-based Electronic Frontier Foundation, a nonprofit that advocates for civil liberties and privacy.

While Maass said more details are needed about the proposal, he questioned a scenario in which license plate information was used for identification purposes.

The EFF has an ongoing lawsuit against the Los Angeles Police Department and the Los Angeles County Sheriff’s Department over the collection of license plate data.

“What happens if you have legitimate reason to be in a neighborhood?” Maass said.

Combating prostitution is a major agenda item for Martinez. The councilwoman last week officially kicked off a new police task force focused on human trafficking in the area.

In a statement Wednesday, Martinez said: “If you aren’t soliciting, you have no reason to worry about finding one of these letters in your mailbox. But if you are, these letters will discourage you from returning. Soliciting for sex in our neighborhoods is not OK.”

Cindy Sower, a Sun Valley business owner, applauded Martinez’s proposal, saying the letters could scare away those men thinking about hiring prostitutes for the first time.

“Let’s say that letter comes in and your wife, your girlfriend or mother gets it,” Sower said. “Maybe it’s a wake-up call.”

John letters have been sent to either suspected or known sex buyers in dozens of areas, according to a report prepared in 2012 for the National Institute of Justice.

Police in Minneapolis, Des Moines and Oakland have “residents record license plate numbers and descriptions of known or suspected johns and their vehicles, and forward the information to police so they can send letters to the alleged offenders (or the registered owners of vehicles appearing to be used by sex buyers),” the report states.