The most senior police officer Britain believes a major terrorist attack on home soil is inevitable.

Metropolitan Police Commissioner Sir John Stevens revealed after the Madrid bombings on March 11, which killed almost 200 people, that police and the security services were "working three times harder than ever".

Sir John said: "We know we have stopped terrorist attacks happening in London but, as the Prime Minister and Home Secretary have said, there is an inevitability that an attack will get through. But my job is to make sure that does not happen.

"If it's necessary to bring in the Army to protect parts of London we will make that decision or if we have to bring in officers from outside the Met, then I'll do that."

Home Secretary David Blunkett appeared to rebuke Sir John and said he was concerned about the "knock-on effect" of such stark warnings.

Mr Blunkett rejected the suggestion a British attack was an inevitability, saying: "I have not used that phrase.

"Of course, I cannot guarantee we will not have an attack, because of the nature of the threat and dangers of suicide bombers.

"There has to be a balance between telling the truth and reassurance. Otherwise people are jumpy without it having a good effect."

The country has been on the second-highest level of security alert since last November. Sir John revealed his force had made 520 terrorism-related arrests since the New York attacks on September 11, 2001.

Sussex Police have drawn up £2 million plans, under the code name Operation Otter, for the Labour Party conference in Brighton this September.

All police leave has been cancelled and there will be a security "island" with intelligence agents vetting those entering the Grand Hotel and the Brighton Centre.