An increasing number of forced marriages are being conducted over Skype, two charities said on Monday.

Imams are conducting ceremonies over the internet or on the phone for British girls as young as 11, meaning they can remotely marry men abroad, according to the charity Freedom which campaigns against forced marriage.

In one case, an 11-year-old girl from London was married on Skype to a 25-year-old man in Bangladesh, said Freedom's founder Aneeta Prem.

The girl hadn't understood the significance of the Skype call at the time and contacted Freedom after reading a book about forced marriage by Prem that her brother was given at school.

"As soon as she was old enough the family were planning to take her out to consummate the marriage," Prem said.

"She had been pulled out of school and was being taught to be a housewife."

Prem said only a handful of internet marriages had come to light so far, but the number was increasing. They are attractive to traditional Islamic families because they are instantaneous and are less likely to get caught, according to Prem.

The practice is illegal under UK law and the Home Office have said perpetrators could be prosecuted.

"Forced marriage is an abhorrent practice which is totally unacceptable and will not be tolerated in the UK, regardless of how it takes place," a spokesperson said.

Karma Nirvana, another British charity which campaigns against forced marriage, said it had received calls to its helpline from girls who had been married off over the phone.

The girls often don't realise the phone call is a marriage ceremony until their family tells them afterwards.

"Some victims have contacted us and said: 'I've been married over the phone because my family think that I'm shaming them,'" said Karma Nirvana founder Jasvinder Sanghera.

"The quickest way to marry someone off is not going to be taking them out to Pakistan, India, or wherever, the quickest way to get them into marriage is going to be over the phone."

Karma Nirvana helpline manager Priya Manota estimated thousands of British girls under the age of 18 are being forced into marriage every year.

Britain's Forced Marriage Unit confirmed it had come across cases of forced marriage being conducted by Skype.

Additional reporting from Reuters.