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The number of under-18s detained under the Terrorism Act when entering or leaving the UK has more than tripled over two years, new figures suggest.

Forty-six were detained in 2015, compared with 13 in 2013, with the youngest aged only 13.

Attempts to prevent young people travelling to Syria could be a factor in the rise, said independent terror legislation reviewer David Anderson QC.

The Home Office has not yet responded to a request for comment.

Six-hour questioning

The BBC's Victoria Derbyshire programme obtained the figures through a Freedom of Information request to the National Police Chiefs' Council.

The data relates to schedule 7 of the Terrorism Act 2000, which grants police special powers to question and detain for up to six hours any individual passing through a UK port, airport, international rail terminal or border area.

The figures show those identifying as Asian or Asian British were six times more likely to be detained than those who were white.

Under the Act, there is no requirement for an officer to have a "reasonable suspicion" that someone is involved with terrorism before they stop an individual. Failure to co-operate with officers can result in three months in prison, a fine or both.

A further 190 under-18s were examined, but not detained, by police between July 2015 and March 2016 - allowing them to be questioned and searched for a maximum of an hour.

Sabah's story

Sabah Choudhry, 21, from London, was stopped and questioned by police while travelling to Turkey with friends last summer.

"It's a really uneasy feeling. I understand why they did it, because at that time there were lots of young people going to Turkey, possibly trying to get into Syria. But it feels like collective punishment for the actions of a few Muslims. I think it was stereotyping," she explained.

Ms Choudhry says one of her relatives was stopped on a separate trip while under the age of 18, and "questioned for more than an hour".

"She wears the hijab, so she was visibly Muslim. She was asked questions on whether her family let her go to university and if she has to have an arranged marriage.

"She is so embarrassed to talk about it," she added.

'Essential power'

David Anderson QC, the independent reviewer of terrorism legislation, said schedule 7 was "an essential power in the fight against terrorism", through which important intelligence could be gathered.

The threefold rise, he said, may partly be the result of police stopping unaccompanied minors on outbound flights who they believe could be travelling on to Syria.

The obligation, since August 2014, to detain anyone questioned for over an hour may also be a factor, he added.

Mr Anderson acknowledged that innocent people may be stopped as a result of the Act, which could be "frustrating".

The Victoria Derbyshire programme is broadcast on weekdays between 09:00 and 11:00 on BBC Two and the BBC News Channel.