It is "only a matter of time" before a terror attack is launched against a city in the West, the director general of MI5 warned yesterday.

In her first public speech since she became head of the service last year, Eliza Manningham-Buller said that renegade scientists have provided al Qaeda with the technical knowledge to develop chemical, biological and nuclear weapons

"The threat from international terrorism is with us for a long time," she told a conference in London.

"If this is a war that can be won, it's not going to be won soon."

She added: "However successful we are, whatever resources the government gives us, there is no such thing as complete security."