Mayor Boris Johnson told ITV London that the law now needs to be able to expose people performing FGM.
Alimatu Dimonekene told ITV London about her FGM ordeal at the age of 16.
Alimatu says she was forcibly held down by an aunt who sat on her at her grandmother's house in Sierra Leone. @itvlondon
Johnson says London maternity units are seeing 200 cases a year. @itvlondon
Boris Johnson says FGM is "Unbelievable in the 21st Century" @itvlondon
Boris: This can be stopped. I want to see the first successful prosecution. @itvlondon
The World Health Organisation describe female genital mutilation (FGM) as "procedures that involve partial or total removal of the external female genitalia, or other injury to the female genital organs for non-medical reasons"
The government says FGM is usually carried out on girls between infancy and age 15, with the majority of cases occurring between the ages of 5 and 8 years.
Because of the hidden nature of the crime, it is difficult to estimate FGM's prevalence, but a study based on 2001 census data in England and Wales estimated that 23,000 girls under the age of 15 could be at risk of FGM each year; and nearly 66,000 women are living with its consequences.
Battersea MP and Health Minister Jane Ellison signed the pledge with Boris Johnson today.
Ellison: No one can be any doubt this nation wants to tackle FGM. @itvlondon