About 250 British-based extremists who went to fight in the civil war in Syria are thought to have returned to this country.
Ministers have been told that over the past two years more than 400 Britons went to Syria to train and fight as paramilitaries, and just over half are believed to have now come back.
Some 16 people were arrested on suspicion of terrorism offences related to Syria in January compared with 24 arrests in the whole of last year.
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, (pictured above) 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 killed in the civil war.
Two British brothers, named in reports as Akra and Mohamed Sebah from north London, are believed to have died in battle in the war-torn country in September.
And in June Channel 4 News broadcast exclusive footage of Ibrahim al-Mazwagi, the first British fighter to die in Syria.
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.