A biological, chemical or nuclear terrorist attack on a Western city is "only a matter of time", the head of MI5 said yesterday.

Eliza Manningham-Buller said: "We are faced with a realistic possibility of a form of unconventional attack that could include chemical, biological, radiological or nuclear."

The director-general of MI5 said intelligence suggested that "renegade scientists" had given terrorist groups the information they needed to create such weapons and that they would become more sophisticated.

But she added that conventional bombs and suicide bomb attacks remained their preferred weapons.

"They (al Qaida) still remain an organisation capable of deadly terrorist attacks," she said.

"The threat from international terrorism is with us for a good long time.

"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.

"Breaking the link between terrorism and religious ideology is difficult."

Ms Manningham-Buller was making her first on-the-record speech since taking on her new job in October last year.

Since then, she has been involved in countering the ricin plot and was involved a review of vulnerable targets which led to a ring of concrete protection being placed around the Houses of Parliament.

Britain was put on its second highest security alert last month.