Prime Minister David Cameron told MPs the Government will not stand for extremists or "groups like the English Defence League who try to demonise Islam and try and stoke up anti-Muslim hatred".
Mr Cameron said in a statement to the House of Commons, "It is as if for some young people there is a conveyor belt to radicalisation that has poisoned their minds".
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Some 66 people have been killed after the plane they were travelling on crashed in southern Iran.
The letter calls for an end to harassment, abuse, and impunity in a world "ripe for change".
Police say that anyone who sees Whitehouse should not approach him, but instead call them on 999 and quote incident 23 of 18 February.