Two 17-year-old boys from north-west London have been detained in Turkey along with a third male aged 19, the Met Police have said.
In a statement the force said: "On Friday 13 March officers from SO15 Counter Terrorism Command were made aware that two 17-year-old boys from north-west London had gone missing and were believed to be travelling to Syria.
"Enquiries revealed that they had travelled with a third male, aged 19 years.
"Officers alerted the Turkish authorities who were able to intercept all three males, preventing travel to Syria. They remain in detention in Turkey. The families have been kept informed of developments."
Three British teenagers have been detained in Istanbul while allegedly planning to cross to Syria to join Islamic state, Turkish officials have said.
The teenagers have not been named and were seized on Friday, sources told Reuters.
Turkish authorities are in touch with their British counterparts to arrange their deportation to Britain this week.
Aircraft from the Afghanistan campaign roared over the parade in their own salute to mark the end of the conflict.
RAF Chinook helicopters, Hercules transport planes and Tornado attack jets flew over St Paul's Cathedral as the parade through the City of London to Guildhall took place.
War veterans, serving soldiers and families joined together to sing the national anthem at a ceremony honouring those who fought in Afghanistan.
Hundreds of people gathered at St Paul's Cathedral for a service marking the end of 13 years of UK military operations in Afghanistan today.Read the full story ›
British and coalition troops took part in an Afghanistan commemoration service in Kabul today as a similar event took place at St Paul's Cathedral in London.
Former Labour Prime Ministers Tony Blair and Gordon Brown have arrived at a special service honouring British forces personnel who died during the Afghanistan campaign and veterans who survived the conflict.
Brown arrived shortly after Blair at St Paul's Cathedral, London, where the service is being held.
RAF Chinooks - used throughout the Afghanistan operation - carried out a flypast over St Paul's Cathedral as part of a ceremony marking the end of the conflict.
British service men and women are marching through London this lunchtime in a parade honouring those who served during the conflict in Afghanistan, and marking the official end of the operation.
The last remaining troops were lifted out of the Allied base in October.
The Prince of Wales was due to take a salute before the parade stepped off.
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A cross made of shell casings which adorned the wall of the main Allied base in Afghanistan has been rededicated at a special commemorative service honouring those who served in the conflict.
The cross was blessed by the Archbishop of Canterbury and will feature on a new Bastion Memorial Wall at the National Memorial Arboretum in Staffordshire.
The ceremony began with a prayer from the Dean of St Paul's Cathedral, the Very Reverent Dr David Ison.
We commemorate with sorrow and regret those who have died in these operations and pray for their families and friends and for all who mourn their loss.
We remember those who have been injured in body, mind and spirit and we pray for their continued wellbeing and for those who especially support them.