The Metropolitan Police chief said a campaign launched to identify possible victims of female genital mutilation (FGM) "warns" people of the law.
Sir Bernard Hogan-Howe said: "They have an option to stop - the fact they bought a ticket to me is irrelevant.
"They should not be committing a serious attack on a child and they should not be breaking the law."
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Loudspeakers at the border have been switched off as a gesture of goodwill ahead of a meeting between the leaders of North and South Korea.
Liam Colgan vanished on February 10 while celebrating his brother’s stag-do in the German city.
Salah Abdeslam and an accomplice were handed the maximum sentence permissible over the incident in March 2016.