A mother and son were arrested when police raided a brothel after gaining entry by pretending to be punters.
Police were responding to complaints about antisocial behaviour and men visiting an address in Rushey Green late at night.
Officers knocked on the door and pretended to be punters, and had also obtained a warrant.
They detained a man and three women, two of whom were sex workers.
The man and the older woman, who was his mother, were arrested on suspicion of inciting prostitution for financial gain and are currently at a south London police station being questioned.
A spokesman for Evelyn and New Cross safer neighbourhoods team said: "Both the female workers were spoken to and given help and advice on staying safe whilst working in an indoor sex environment.
"We discussed the options that they had about leaving the industry and gave them a booklet to assist them staying safe.
"One of the workers was arrested for immigration offences and the other was allowed to leave the premises.
"Details of the visit are being collated and are to be passed onto Lewisham Council for their follow-up in closing the address down."