The families of two of three teenagers, from north-west London, arrested over terror offences reportedly raised the alarm to police, leading to the trio being detained by officials in Turkey as they tried to travel to Syria.

Upon their return to the UK, counter-terrorism police arrested the trio, a man aged 19 and two 17-year-old boys on Saturday night on suspicion of preparing terrorist acts. The teenagers have been bailed as the investigation continues, police said.

On Friday police were made aware the two boys had gone missing and were believed to be travelling to Syria. Inquiries revealed they had travelled with a third person, the 19-year-old man.

The families of the younger boys alerted officers when the pair did not return from Friday prayers, according to reports.

A Met police spokesman said: "Officers alerted the Turkish authorities who were able to intercept all three males, preventing travel to Syria.

"On Saturday, March 14 the three males returned to the UK and at approximately 11.10pm were arrested on suspicion of preparation of terrorist acts contrary to section five of the Terrorism Act 2006.

"They have been bailed to return to a central London police station pending further enquiries."

The trio are reported to have travelled to Spain before taking a flight to Turkey - a route taken by Hayat Boumedienne, the widow of one of the terrorists who carried out the Charlie Hebdo attack in Paris in January - though the Foreign Office could not confirm this.

It comes just weeks after an international manhunt was launched to find three London schoolgirls who travelled to Istanbul on their journey to Syria.

However, Shamima Begum, 15, Amira Abase, also 15, and Kadiza Sultana, 16, who are pupils at Bethnal Green Academy, are now feared to have reached the conflict zone to become so-called "jihadi brides" with Islamic State.

The families of the three girls released a statement on Sunday appealing for them to come home immediately.

Saying they felt their loss "more acutely" on Mother's Day, the families also criticised authorities, saying: "With respect to the disappearance of our children we have been disappointed by the handling of this matter by the school, Met Police and the local authority, all of whom we feel failed to act appropriately and pass on vital information to us or indeed between each other."

About 600 Britons are believed to have travelled to Syria and Iraq since the conflict began, according to Scotland Yard assistant commissioner Mark Rowley, the national policing lead for counter-terrorism.

Speaking last month, he added that around half are believed to have returned to the UK.

MP Keith Vaz, who chairs the Home Affairs Select Committee, said the latest arrests suggest the flow of young people intending to travel to Syria is "on a much larger scale than we envisaged".

"I welcome the action that has been taken by the Turkish authorities," he said.

"We need to prevent people going in the first place and that is why parents need to be vigilant but we also need co-operation from the Turkish authorities in order to stop them from going further.

"We need to be vigilant. Clearly this flight of young people to Turkey in order to go to Syria is on a much larger scale than we envisaged."

Concerns about how Turkish authorities dealt with the disappearance of the three missing girls last month, raised by their families, proved how important it is to "act quickly", he added.

Sajda Mughal, director of the JAN Trust which offers workshops to Muslim parents on the dangers of online radicalisation, said: "The detaining of the boys in Istanbul highlights now a more joined up approach of working which is what is required if anyone has left the UK.

"The route to Syria is through Turkey and this case has shown that the agencies managed to stop them from getting into Syria as they communicated with each other.

"Whilst these cases are coming out in the media and people become aware, this is not enough to tackle the issue of radicalisation.

"What is required is that Muslim homes across the UK must be educated on the issue and online radicalisation to take ownership and so that they can safeguard their children."