LONDON - Female genital mutilation, commonly associated with parts of Africa and the Middle East, is becoming a growing problem in Britain despite efforts to stamp it out. London's Metropolitan Police, Britain's largest police force, hopes a campaign beginning on Wednesday will highlight that the practice is a crime here.

To make their point, police are offering a $40,000 reward for information leading to Britain's first prosecution for female genital mutilation, Detective Chief Superintendent Alastair Jeffrey said.

In Britain, the problem mostly involves first-generation immigrants from Africa and the Middle East.

Police say they don't have comprehensive statistics about the number of victims. But midwife Comfort Momoh, who specializes in treating them at London hospitals and clinics and who works with police, told the news conference she treats 400 to 500 victims every year.

Arranging or carrying out the procedure _ in Britain or abroad _ is a criminal offense punishable by up to 14 years in prison, but no one has been prosecuted since it was banned under British law in 2003, Jeffrey said. Police estimate up to 66,000 girls in Britain face the risk of genital mutilation.

"The timing of this campaign is for one good reason: so we can get in before the summer holidays, a time when young girls are taken abroad and subjected to genital mutilation," he told a news conference Tuesday.

Mutilated infants, girls and women face irreversible lifelong health risks _ both physically and mentally, according to UNICEF and other charity groups.

Authorities believe the number of genital mutilation cases peaks in the summer, because the extended school vacation gives girls more time to recover _ thereby making it easier for those responsible to cover up their actions.

Female genital mutilation usually involves the removal of the clitoris and other parts of female genitalia. Those who practice it say it tames a girl's sexual desire and maintains her honor.

It is practiced by Muslims and Christians alike, deeply rooted in the Nile Valley region and parts of sub-Saharan African, and is also done in Yemen and Oman. Through migration, the practice has spread to Western countries like Britain.

U.S. federal law specifically bans the practice.

Between 100 million and 140 million women are believed to have been subjected to the practice in Africa and an additional 3 million girls face the threat of female genital mutilation every year, according to UNICEF.

Detective Inspector Carol Hamilton, who has been investigating the practice since 2004, said some immigrants in Britain may bring practitioners from their home country to mutilate several children because it is cheaper.

She said children not only suffer terrible physical injuries, but can also be left emotionally scarred.

Salimata Badji-Knight was mutilated when she was 4 years old in her native Senegal.

Now married and living in London, she fears she may not be able to have children because of the procedure. She hopes that by sharing her experiences she can prevent parents from subjecting their daughters to similar abuse.

"Why do they need to go and mutilate a young innocent person without her knowing what is going to happen, just for culture?" Badji-Knight said. "It does not add up for me."

Somali-born supermodel Waris Dirie survived a traditional form of the practice that kills hundreds of girls each year.

French President Nicolas Sarkozy is set to present the "Chevalier de la Legion d'honneur" to her on Thursday for her work as a leading critic of female genital mutilation, which has seen her tour parts of Africa to speak out against the practice.