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.