Nearly 4,000 women and girls have been treated for female genital mutilation (FGM) in London's hospitals since 2009, according to new figures.
Guy's and St Thomas' NHS Foundation Trust treated the most FGM patients, with 1,146 seen over the past five years.
The data was obtained through Freedom of Information requests to NHS Trusts by BBC London 94.9.
London's mayor Boris Johnson said the practice was "absolutely intolerable".
Forty-one hospital trusts were contacted by the BBC with most supplying figures. Six said they did not currently record figures on FGM, while four did not respond.
The remaining 31 trusts reported that a total of 3,939 FGM patients had been treated in London hospitals.
St George's Healthcare NHS Trust treated the second largest number of patients, with a total of 795 seen between 2009 and 2013, eight of whom were born in the UK.
The trust also revealed six girls under the age of 18 had been treated.
Mr Johnson said he was committed to delivering a pilot initiative aimed at improving the way agencies in the capital identify and respond to FGM.
"This is a crime basically outlawed in the early-mid 1980s and yet, unlike France, we have not had one single successful prosecution for what is unquestionably a completely barbaric crime," he said.
"It's time to stop being so nervous, so gingerly and hesitant. This is something that is absolutely intolerable in a place like London."
FGM is most common among some African, Asian and Middle Eastern communities.
But campaigners say there is a lack of knowledge about how prevalent the practice is and this is hampering social services and the police in the collection of evidence.
Dr Phoebe Abe, a Hillingdon GP, runs a free weekly FGM clinic where she is currently treating 60 women, eight of whom are under 17.
"These women have adhesions, vaginal infections, pelvic inflammatory infections, urine infections - for years they have been having this problems," she said.
Last month the Department of Health announced that, from April, all NHS hospitals will be able to record if a patient has undergone FGM or if there is a family history of it.
By September, all acute hospitals will have to report this data to the department on a monthly basis.
At least 66,000 girls and women in the UK are believed to be victims of FGM.
The children's charity, NSPCC, set up a 24-hour FGM helpline last year.