Charity advises women and young girls to set off airport metal detectors to give them more time to seek help from authorities

A number of women and girls at risk of forced marriage have avoided going abroad by concealing spoons in their underwear at airport security, according to a campaign group.

Karma Nirvana, a Derby-based charity that supports victims of forced marriage, advises people who ring its helpline to hide a spoon in order to set off metal detectors at British airports. The group says that its recommendation has prevented some women from being spirited overseas.

Last week ministers warned that young people were at the highest risk of being taken abroad for a forced marriage during the school holidays. The government's forced marriage unit received 400 reports between June and August last year, out of an annual total of 1,500.

No one knows for sure how many Britons are forced into marriage each year. Estimates range from 1,500 to 5,000. More than a third of those affected are thought to be aged under 16.

Speaking to the AFP news agency, Natasha Rattu, Karma Nirvana's operations manager, said that when worried youngsters ring the charity's helpline, "if they don't know exactly when it may happen or if it's going to happen, we advise them to put a spoon in their underwear.

"When they go through security, it will highlight this object in a private area and, if 16 or over, they will be taken to a safe space where they have that one last opportunity to disclose they're being forced to marry."

The government wants teachers, doctors and airport staff to be conscious of the issue of forced marriages over the summer break.

Aneeta Prem, founder and president of Freedom Charity, an organisation that deals with the prevention of forced marriage through education and training, believes that summer is a crucial time for children and young adults.

"Children go out of people's consciousness over summer because they are away for such a long time," she told the Guardian. "The victim may think they are going away to a family wedding, not knowing it is actually their wedding. And when they go they are often gone for a long time and don't come back until they are pregnant."

Campaigners fear official statistics on the number of forced marriages of UK citizens are just the tip of the iceberg, partly because children do not want to report their parents to the authorities or have little idea where to go for help.

Prem said: "Nobody knows what the true figure is because so many young victims are terrified of coming forward. But it is definitely much, much higher than what is reported."

Freedom Charity has produced an app for potential victims of forced marriage or other abuse. It is also aimed at friends of those women who may be at risk and professionals such as teachers. Since the app was launched in March, more than 1,000 people have contacted Freedom Charity using the technology. The charity also has a 24-hour helpline.

The Karma Nirvana charity usually fields 6,500 calls a year from around Britain. This year, it has already reached that number.