British police will ask women to help persuade young people not to go out to fight in Syria, in a new campaign to stem an exodus which authorities fear could breed militants at home.
Hundreds of Britons are thought to have flown out to join rebels fighting the forces of Syrian President Bashar al-Assad.
Senior police officers and security chiefs have said they are worried the fighters could return to the UK radicalised by hardline Islamists and carry out attacks on British soil.
Britain's counter-terrorism police said they were going to appeal to women from "affected communities".
"We are increasingly concerned about the numbers of young people who have or are intending ... to join the conflict," Helen Ball, National Coordinator for Counter Terrorism Policing, said in a statement ahead of Thursday's launch.
"We want to ensure that people, particularly women, who are concerned about their loved ones are given enough information about what they can do to prevent this from happening."
Last week, a teenager from southern England was killed fighting in Syria. Police said anyone who did make it back home risked arrest.
About 40 people have been detained in the first three months of this year, compared to 25 during the whole of 2013.
One of those arrested this year was former Guantanamo Bay prisoner Moazzam Begg who was charged in March with terrorism offences although his supporters say he was merely carrying out humanitarian work.
Ball said they recognised that not all who went to Syria did so to fight, and said the campaign aimed to inform people how they could help those in need in Syria "safely and legally".
Andrew Parker, the Director General of MI5's domestic spy agency, said in October a growing proportion of his casework involved Syria, "mostly concerning individuals from the UK who have travelled to fight there or who aspire to do so".
Parker led MI5's response to the 2005 suicide bomb attacks on London which killed 52 civilians. Some of the bombers are thought to have received training in al Qaeda camps in Afghanistan and Pakistan.