Sir Ian Blair, Commissioner of the Metropolitan Police, said that a single mistake by the bombers had avoided carnage on three Underground trains and a No 26 bus. In the starkest language he has used since 52 people and four bombers died in the 7/7 attacks, Sir Ian spelt out the nature of the terrorist threat facing London.

“This is a campaign we are facing — not a one-off event,” he said. “The second attack on July 21 should not be taken as some indication of weakening of the capability or the resolve of those responsible.

“This is not the B team. These are not amateurs. They made a mistake. They made one mistake. We are very, very lucky. The carnage that would have occurred had those bombs gone off would have at least been the equivalent to those on July 7.”

But he questioned the safety of using a Taser electric stun gun to subdue a suspect for the attempted London bombings. He and his officers could not understand why the Taser was used by West Midlands Police to arrest Yasin Hassan Omar in Birmingham on Wednesday.

He told a BBC television programme on the security crisis: “It is incredibly risky to use a Taser on a suicide bomber because it may have set it off. I can’t imagine why it was used. They are used regularly in London but it sends an electric current into the body. If there is a bomb on the body it sets it off.”

He remained confident that the three missing bombers from July 21 and the support cell behind both attacks would be caught. But there could be further terrorist bombs before the emergency was over.

“We are in a sombre moment and it does remain possible that those at large will strike again. And it does also remain possible that there are other cells that are capable and intent on striking again.” There was no reason to expect terrorists to confine themselves to targeting the transport system, he said.

As the security operation entered its fourth week, it was costing £500,000 a day and his force was so stretched that it had borrowed 150 officers from outside the capital.

At Paddington Green police station a specialist team has begun to interview Omar, 24, the suspected Warren Street bomber held in Birmingham on Wednesday. Sources said that questioning was progressing very slowly. Omar, who was born in Somalia but has lived in Britain since he was 11, was said to have asked for a translator.

He has reportedly sought medical examinations and asked for sickness breaks, claiming that he is suffering nausea and headaches after being shot with the Taser stun. Omar has also alleged that officers struck him around the head when they arrested him. He can be held without charge for 14 days, which police chiefs consider inadequate for full interrogation.

A huge manhunt continues for three accomplices, one of them named as Muktar SaidIbrahim, 27. Arrests have been made and searches are ongoing at addresses in Tooting and Stockwell, South London, for the two other bombers named. Elsewhere, specialist teams are conducting searches in North London, Leeds and Birmingham. Forensic experts are also continuing to examine a large cache of explosive devices and components found in the boot of a car left by the 7/7 bombers at Luton railway station. One device, a picture of which was published by ABC News in the United States, is held in a jar with a crude fuse made up from scores of matchstick heads.

Sir Ian, who also briefed chief constables from around Britain, said that there were many “tired faces” at Scotland Yard. He said that 1,000 officers were involved in the inquiry but that the extent of the threat necessitated a strengthening of resources. “We will have to strengthen, in the next year the firearms capability of the Metropolitan Police,” he said. “It is not in any sense inadequate but we have to give people rest.”

Sir Ian again defended the “shoot-to-kill” policy that led to the mistaken shooting of Jean Charles de Menezes, a Brazilian electrician, a week ago.

Sir Ian said that the police “clearly thought they were facing a suicide bomber and they were racing towards a suicide bomb and certain death”. There was no foolproof way of dealing with a suicide bomber.

HOTLINES FREE

Casualty information hotlines are to be made free to dial after the bombings. The 0870 numbers used after July 7 were criticised for costing up to 40p a minute. Future casualty bureaux will use 0800 numbers, which are free to most users.

WHAT SIR IAN SAID YESTERDAY

‘This is not the B team. These are not amateurs. They made a mistake. They made one mistake’

‘We are very, very lucky. The carnage that would have occurred had those bombs gone off would have at least been the equivalent to those on July 7’

‘We are in a sombre moment and it does remain possible that those at large will strike again . . . and that there are other cells that are capable and intent on striking again’

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