And in a stark warning, MI5 chief Andrew Parker said: “Although we and our partners try our utmost, we know we cannot hope to stop everything.”
He said that the threat of a “mass casualty attack” was growing and that intelligence pointed to the existence of specific plots.
Parker, who was appointed in March 2013, told an audience at MI5’s headquarters in London that although three terrorist plots had been foiled in recent months, it was unfortunately possible that one could succeed.
MI5 officers have increased surveillance of fanatics who had travelled to Syria whom they fear may launch copycat attacks, after 12 people were slaughtered on Wednesday by gunmen, thought to be part of an al-Qaeda group, at the satirical Charlie Hebdo magazine in Paris.
Parker warned the UK was facing "more complex and ambitious plots" by extremists. The Paris outrage was “a terrible reminder of the intentions of those who wish us harm,” he said.
He said if MI5 hadn’t foiled the terrorist plots, “deaths would certainly have resulted”.
He said the number of “crude but potentially deadly plots” MI5 was facing had also increased, especially those by “lone wolf” terrorists.
“Such attacks are inherently harder for intelligence agencies to detect,” Parker said.
“They are often the work of volatile individuals, motivated by terrorist propaganda rather than working as part of sophisticated networks.
“They often act spontaneously or after very short periods of prior planning.”