The head of MI5 warned yesterday that it was "only a matter of time" before al-Qa'ida terrorists carried out a nuclear, chemical, or biological attack on a Western city.
Eliza Manningham-Buller, the director general of the Security Service, said "renegade scientists" - understood to be from Pakistan - had given Islamic extremists information to create weapons of mass destruction, such as "dirty bombs", and that they would become ever more sophisticated.
In her first public speech since taking over at MI5 in October, she told the Royal United Services Institute in London: "We are faced with a realistic possibility of a form of unconventional attack that could include chemical, biological, radiological or nuclear (CBRN).
"It is only a matter of time before a crude version of a CBRN is launched on a Western city, and it is only a matter of time before the crude version becomes more sophisticated."
She also disclosed that there was concern that the dissident Irish terrorist group, the Real IRA, would carry out another bombing campaign on the British mainland.
Ms Manningham-Buller, 54, said: "Al-Qa'ida has the ambition to carry out unconventional attacks against the West. We know renegade scientists have co-operated with al-Qa'ida and provided them with some of the knowledge they need to develop these weapons."
She added that the discovery of traces of the deadly poison ricin at a flat in Wood Green, north London, demonstrated an interest by terrorists in chemical weapons. But she stressed the most likely type of attack in the West was from conventional bombs and suicide bombers.
She also disclosed that a new terrorism warning centre, based at MI5 headquarters in central London, is receiving about 150 pieces of intelligence concerning possible terrorist threats every day.
Britain is on a heightened state of alert against terrorist attacks. Security has been tightened at airports and other possible targets, such as the Houses of Parliament, and extra police officers have been placed on patrol. Police forces have been trained in how to deal with chemical, nuclear or biological attacks.
But, she said: "If this is a war that can be won, it is not going to be won soon. The supply of potential terrorists among extreme elements is unlikely to diminish."
Talking about the impact of the 11 September attacks in America, she said: "During the summer of 2001, the UK agencies knew that attacks, probably against US interests, were imminent but their nature and target was unknown. What shocked us all was the scale and devastation of the attacks."
She told the conference that al-Qa'ida's willingness to strike "tourists and members of the pubic" and to use suicide bombers made it difficult to stop, despite increased international co-operation and greater resources. MI5 has recently recruited 200 new officers and dedicates about a third of its resources to counter-terrorism.
But she said calls for a root and branch review of Britain's security services risked ignoring their positive work, adding: "The system works." She said Britain had "unrivalled expertise" in combating terrorism, including a "robust and well established system in handling the threat".
Countering Irish Republican terrorism remains one of MI5's top priorities. To illustrate the fears of further outrages on the British mainland, Ms Manningham-Buller said that the Real IRA carried out the last terror attack in Britain, and highlighted the seizure of a 250kg (600lb) bomb in the back of a van in Derry City, Co Londonderry, on Sunday.