It is only a matter of time before terrorists launch a chemical, biological or nuclear attack on a Western city, the head of MI5 has warned.
Ms Manningham-Buller: al-Qaeda still a threat
Eliza Manningham-Buller told intelligence experts some form of unconventional attack was a "realistic possibility" as the war against terror continued.
Intelligence suggested "renegade scientists" had given terror groups information needed to create chemical, biological, radiological and nuclear (CBRN) weapons.
"It is only a matter of time before a crude version of such an attack is launched at a major Western city," she said.
She also warned in her unprecedented speech in London that the threat from international terrorism would be "with us for a good long time".
Her speech comes as the UK remains on the second highest level of alert.
Home Secretary David Blunkett agreed there was a known threat.
This was why new legislation had been introduced and why the government was backing MI5 with "considerable" resources, he said.
But he continued: "We know in the end we can't give 100% assurances."
In her speech, Ms Manningham-Buller highlighted the al-Qaeda terror network as "the first truly global threat".
Recent fatal attacks in Riyadh, Saudi Arabia, and Casablanca, Morocco, proved it and other groups still posed a "potent threat".
"If this is a war that can be won, it is not going to be won soon," she said in her first public speech since becoming MI5's director general in October.
"The supply of potential terrorists among extreme elements is unlikely to diminish.
"Breaking the link between terrorism and religious ideology is difficult."
Ms Manningham-Buller told the conference at the Royal United Services Institute that despite the threat of such an attack, conventional bombs and suicide bombers remained the preferred weapons of terror groups.
Garth Whitty, of the Royal United Services Institute, said the fact the director general had been prepared to go on record with her comments was extremely important and should be taken at "face value".
Since taking up her post, Ms Manningham-Buller has been involved in an investigation into the poison ricin.
She has also participated in an a review of vulnerable targets in London which led to a ring of concrete protection being placed around the Houses of Parliament.