Andrew Parker, head of MI5, said that militants are planning mass casualty attacks, and the threat was “on a scale and at a tempo that I have not seen before in my career”.
His remarks coincide with Conservative prime minister David Cameron’s plans to bolster the powers of spies and police.
“(IS) uses the full range of modern communications tools to spread its message of hate, and to inspire extremists, sometimes as young as their teens, to conduct attacks in whatever way they can,” he said.
Intelligence chiefs and Cameron have argued for years that the security agencies need more powers to address the threat and prevent another attack on the scale of the London suicide bombings in 2005 when four British Islamists killed 52 people.
Opposition to more surveillance is widespread, including from within Cameron’s Conservative party, fuelled in part by former US spy contractor Edward Snowden who suggested US and British spies were conducting mass monitoring of communications.
Privacy and human rights campaigners are also fiercely opposed to measures they say are an assault on freedoms.
The proposed new laws are expected to begin their passage through Britain’s parliament next week.
Islamic State is planning 'mass casualty' attacks in Britain, Andrew Parker, the the head of MI5 has warned. pic.twitter.com/MxRnPguFoW— Philip Sime (@PhilipSime) October 29, 2015
An effort in 2012 to beef up the reach of the security services was blocked by the Conservatives’ then-coalition partners, the Liberal Democrats.
“I hope that the public debate will be a mature one ... not characterised by ill- informed accusations of ‘mass surveillance’, or other such lazy two-worded tags,” Parker said.
Cameron now runs a majority government, which should allow him to pass the latest proposals. But there are questions over whether the upper chamber of parliament, where Cameron does not hold a majority, will try to get them watered them down.
Last August, Britain raised its terrorism threat level to ‘severe’, the second- highest category, meaning an attack was ‘highly likely’.
It was largely due to the danger the authorities say is posed by IS fighters and the hundreds of Britons who have joined them.
Parker said a growing proportion of online communications could not be intercepted by his agency and that service providers had “an ethical responsibility” to co-operate with security agencies.
Giving a speech in London, Mr Parker said six attempts at terrorist atrocities in the UK have been thwarted in the last year.
“It may not yet have reached the high water mark and, despite the successes we have had, we can never be confident of stopping everything,” he said, citing the Tunisia beach massacre in June as an “appalling reminder of the threat”.
He spoke of a “three dimensional threat”, at home, overseas and online, with an increasing proportion of the agency’s casework linked to Syria and IS.