Richard Barrett, a former head of counterterrorism at the MI6 intelligence agency, told the BBC that keeping track of all those who returned would be impossible.
British officials have estimated that between 400 and 500 Britons have travelled to Syria to fight, and some may have crossed into Iraq.
A video released online last week showed three men identified as British exhorting compatriots to join the militant group Islamic State of Iraq and the Levant, or ISIL.
Police counterterrorism official Peter Fahy warned that slick ISIL recruiting videos made violent jihad look like a glamorous adventure.
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