TV star Alex Kingston made some surprise discoveries about her family past while using the borough's heritage service.
The ER and Moll Flanders star was researching her family history for the BBC's 'Who Do You Think You Are' series, and to her surprise found her relative ran a brothel.
Ms Kingston, of Epsom, visited the heritage service in Battersea Library, because her great grandfather used to live in the borough.
She was able to trace her family back hundreds of years, unearthing her Jewish roots and finding her great, great, great, great grandmother Elizabeth Braham once ran "a house of ill-repute" in Westminster.
Heritage officer, Ruth Macleod, who appears in the programme, said: "We were able to locate her great grandfather and other members of her family from census records, and armed with those details she was able to start her journey back through history.
"This is something we do on a fairly regular basis for people who want to find out more about their backgrounds and the lives of their ancestors.
"The historical documents we hold are often the key to unlocking some amazing family secrets."
Kingston, who was married to fellow Brit actor Ralph Fiennes, is the eldest of three daughters born to a butcher and his German wife.
The programme was screened on BBC1 at 9pm on September 19.
For more information on Wandsworth Heritage Service call 020 8871 7753 or email firstname.lastname@example.org