Project Azure fears up to 6,500 girls in London are at risk
Police investigating the practice of female circumcision in the UK have expressed anger over being unable to get enough evidence for a prosecution.
But they said 45 girls in the UK, aged between five months and 16, were saved from the act since March last year.
Despite new legislation outlawing the practice, no-one has been convicted.
The Met's Project Azure, which tackles the problem, is offering a £20,000 reward to bring the first person in the UK to justice.
Female circumcision is believed to reduce a woman's desire for sex, and therefore sex outside marriage, and can be carried out on girls as young as four.
Police fear up to 6,500 girls in London are at risk from a tradition that can be traced back to many African nations.
Most at risk
They have identified early summer as the time when young girls are most at risk, because there is sufficient time for them to recover after the attack before school restarts.
Family members will take victims abroad or even club together with others to pay for a mutilator to travel to London.
Det Supt Caroline Bates, of the Met's child abuse investigation command, said: "The awareness raising has been good and we have had intelligence and some operations where we have followed up with surveillance.
"But we have been unable to identify anybody that we can hold criminally responsible.
"Some of that comes down to the fact that we can't age the injuries. The child had been mutilated, but we can't confirm if it was prior or post the legislation."
Earlier this week author Ruth Rendell accused ministers of failing to protect young women.
Baroness Rendell helped introduce a 2004 law that made it an offence to send a child abroad for the procedure.
Anyone involved now faces a 14-year prison term.
She said: "I was absolutely horrified. It is so widespread and such a dreadful thing to do.
"I see it as sort of the ultimate subjugation of women."