It is estimated as many as 400 British nationals are fighting with Sunni militant groups. The Prime Minister pledged to do "absolutely everything we can to keep our people safe".
At a No 10 press conference, he denied the Coalition was restoring diplomatic links with neighbouring Iran simply to win Tehran's help to quell the spread of the Islamic State of Iraq and the Levant (Isis) militants. "No-one should be in any doubt that what we see in Syria and now in Iraq in terms of Isis is the most serious threat to Britain's security there is today," declared Mr Cameron.
The comments came after Foreign Secretary William Hague said the circumstances were right to reopen the embassy in the Iranian capital, which was closed after being ransacked by a mob protesting against sanctions in 2011.