More than 1,500 young British fighters may have been recruited by the Islamic State of Iraq and the Levant (Isis) to fight in Iraq and Syria, according to an MP.
Khalid Mahmood, MP for Birmingham Perry Barr, told Sky News that the number of potential jihadists emanating from Britain's shores is much higher than anti-terrorism chief Sir Peter Fahy's figure of 500.
"I imagine 1,500 certainly would be the lower end. If you look across the whole of the country, there's been a number of people going across," said Mahmood.
"Originally you had the British Syrians settled here who wanted to go back and play a part, then you had the Kurdish community, then almost two years ago you had the young British Muslim community going across - so if you add all that up you've got serious figures that we need to look at."
The politician said the UK needed to strengthen its border controls to ensure that radicalised Britons are not returning to the country and posing a threat to national security.
"Those will come back - certainly more than we are saying at the moment. Some of them go out there for nine months, some go out there for a year, some will just go out for six months or so.
"So we need to look at exactly how this is happening, what our border controls are like and we really need to bolster those border controls in order to ensure that we see the people coming through and deal with them."
A newly-released video from the group, entitled "There is No Life Without Jihad", shows three Britons - Abu Muthanna al-Yemeni, Abu Bara al-Hindi and Abu Dujana al-Hindi - pledging to take their holy war to Jordan and Lebanon if ordered by Isis chief Abu Bakr al-Baghdadi.
"We are a state who is implementing the Sharia in both Iraq and the Sham. And look at the soldiers, we understand no borders," says al-Yemeni in the video.
"We have participated in battles in Sham and we will go to Iraq in a few days and we will fight there. We will even go to Lebanon and Jordan with no problems, wherever our Sheikh [Baghdadi] wants to send us."
British Foreign Secretary William Hague has estimated a much lower figure of 400 British nationals who have travelled to Syria to fight for Isis.