Around 250 British-based extremists who went to train and fight in Syria have returned to the UK, it has emerged.
Ministers have been told that over the past two years more than 400 Britons have gone to Syria and it is now thought just over half have returned.
In January alone, 16 people were arrested on suspicion of terrorism offences related to Syria compared with 24 arrests in the whole of last year.
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Syria War In February
A senior Whitehall source said: "The threat from Syria is serious and presents real challenges for intelligence agencies.
"But not all of those returning will start engaging in UK-based attack planning.
"For some, their jihad is done, others will help others travel to Syria, while others will raise funds for fighting."
The number of returnees, first revealed by The Sunday Times, has emerged after details were reported of the first instance of a British-based jihadist staging a suicide attack in Syria.
Abdul Waheed Majeed, 41, is suspected of being responsible for driving a lorry into a jail in Aleppo and detonating a bomb earlier this month. He is among an estimated 20 Britons who have been killed in the fighting in the war-torn state.
Photographs were published last month of two British brothers, named in reports as Akra and Mohamed Sebah from north London, who are believed to have died in battle in the war-torn country in September.
The two men were pictured together smiling and brandishing guns in camouflage gear and were reportedly hailed as "martyrs" and "young British lions" in propaganda messages.
Meanwhile, two women appeared in court last month accused of trying to send 20,000 euro (£16,500) to fund alleged Islamic terrorists fighting in Syria.
Londoners Nabal Masaad, 26, and Amal El-Wahabi, 27, are due to appear at the Old Bailey in May.
Security officials and senior politicians have warned for some time of a potential surge in radicalised foreign fighters returning to the UK from war-torn Syria.
MI5 director-general Andrew Parker told MPs last year that the civil war in Syria has been a magnet for hundreds of British nationals looking for the opportunity for "jihadi" activity, many of whom have come into contact with al Qaida-supporting groups before returning to the UK.
Home Office minister James Brokenshire warned that security concerns linked to Syria were "likely to be with us for the foreseeable future".
"A significant proportion and a growing proportion of the security services work is linked to Syria in some way," he told BBC Radio 4's The World This Weekend.
"This is a big problem that the security services and the police are actively focused on.
"It's why they are vigilant, why they are taking the steps that they are around the border and monitoring travel to and from Syria in the way that they are."
Brokenshire said Syria had become "the number one jihadist destination in the world", and the number of Britons thought to have travelled there to fight was in the "low hundreds".