Young British men and women fighting in Syria is a "huge problem" with an estimated 500 UK citizens joining radicals abroad, Jonathan Russell told Good Morning Britain.
Political liaison officer at the anti-extremism organisation Quilliam Foundation described British men and women going to Somalia or Syria to fight as "the new face of terrorism".
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Security is to be enhanced around Windsor Castle during the Changing Of The Guard following the Westminster terror attack.
A cloudy start to the day with top temperatures of 18C (64F).
Immigration levels may need to rise, David Davis has said as he refused to commit to a cap on the number of EU citizens coming to the UK.