Gay men are being attacked and robbed at cruising' grounds in Croydon by robbers who have sex with them first.
Attackers are luring victims into a false sense of security before robbing or stealing their cars at knifepoint.
Police are unsure how many of these crimes are taking place in the borough because victims are too ashamed to report it.
But this week Croydon's lesbian, gay, bisexual and transgender (LGBT) liaison officer PC Jeff Wade said he believed there were more victims than the police were aware of.
He told the Croydon Guardian: "It's an ongoing thing. I hear of this happening all the time but people are very reluctant to report it.
"Because they have been intimate with the suspects, the victims have been at their most vulnerable.
"They are embarrassed because and have not wanted to go public. Or they might have families and would not want to give evidence in a potential court case.
"We had one man come to us to say he'd been liaising with a chap on Shirley Hills when the guy in question jumped into the victim's car and said, give me a lift'.
"Reluctantly he did and the next thing he knew a knife was pulled out and the guy tried to steal his car.
"These crimes could be happening because the suspect knows there is a great possibility victims are not going to come forward."
A warning about the crimes appeared on the website for Aurora, Croydon's LGBT police consultation group, which said: "There have been similar events in other cruising grounds where someone robs or assaults a cruiser after having had sex.
"This gives them the advantage of gaining the confidence of the victim."
PC Wade wants more people from the LGBT community to put their trust in police, which is why he took part in a recent enrichment day at Croydon College to raise awareness of hate crime and encourage the reporting of it.
To report a hate crime, contact the community safety unit at Croydon on 020 8649 1265. LGBT officers can also be contacted on 0208 649 0213, or email firstname.lastname@example.org