Britain has raised its terror threat level from 'substantial' to 'severe' in response to developments in Syria and Iraq.
A severe threat level means a terrorist attack is considered highly likely.
Prime Minister David Cameron is also promising new powers to deal with Britons who travel to Syria and Iraq to fight with Islamic State militants.
Olly Barratt reports from London.
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Home Secretary Theresa May says terrorist groups in Syria and Iraq are planning attacks against the West.
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Although the severe threat level means an attack is thought to be highly likely, the home secretary says there is no intelligence to suggest one is imminent.
Prime Minister David Cameron says he has no doubt Islamic State militants in Syria and Iraq are targeting the west though.
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Police say hundreds of Britons have traveled to Syria and Iraq, with swathes of both countries now controlled by Islamic State militants.
And Mr Cameron says they should not be allowed back into the UK.
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David Cameron insists the UK is not planning to take part in military action against the Islamic State in Syria and Iraq.
But he says the group poses an unprecedented threat.
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With a Briton thought to have been involved in the killing of US journalist james foley...
and with so many UK citizens traveling to the region to fight....
Police in the UK are appealing for help from the public to help identify people who may be becoming radicalised or are planning to go abroad to join militant groups..
At Regent's Park Mosque in London - young Muslims say there should be a better explanation from government about the threat ISIS pose.
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Britain's terror threat level last changed in July 2011 when it was reduced to 'substantial'
The only stage higher it could go would be to critical - if an attack was expected imminently.
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