Summer marks a peak in reports of young people - mainly girls - being taken abroad on "holiday", not knowing their families are forcing them into a marriage, the Home Office said.
Britain's Forced Marriage Unit dealt with some 1,500 cases last year, a third of them involving children aged under 17, AFP reports. In 18% of cases, the victim was male.
Victims it had dealt with last year were taken to 60 countries. In nearly half the cases the young people were taken to Pakistan. Other countries included Bangladesh (11% of cases), India (8%) and Afghanistan (2.1%).
"The rise in forced marriage reports over the school holidays is shocking," crime prevention minister Jeremy Browne said on Saturday.
"My message to young people who feel they are at risk is, please come forward - you do not have to suffer in silence, there is help available and it can be stopped."
The Forced Marriage Unit is handing out cards this summer providing information for potential victims, telling them to speak to police or airline staff if they believe they are being taken to be married against their will.
The opposition Labour party said the warning might have been more helpful if it was issued before schools broke up for the holidays.
The government announced plans last year to introduce new laws for England and Wales that would see parents sent to jail if they force their children into a marriage.