London's most senior police officers have said they had no indication that a terrorist attack on the city was imminent and they are confident that security was at an "appropriate" level.

"We are not aware of any warning at the moment," Sir Ian Blair, the Metropolitan Police commissioner, told Sky News. The deputy assistant commissioner, Brian Paddick, later confirmed that there had been "no warning".

Mr Paddick said that in recent months the security alert level had been "lowered slightly" in recent months. It was reduced from "severe general" to "substantial".

Michael Evans, Defence Editor of The Times, said that without specific intelligence it would be hard for the authorities to prepare. "We’re on a fairly high [security] status anyway, but there had been no specific warning.

"They get lot of bits and pieces but unless you get a precise piece of information saying there is going to be an attack on July 7, it is very difficult to do anything with it."

He said that Israeli officials had denied reports that they recieved a warning about an attack immediately before the explosions.

"Al-Qaeda is very, very clever at long-term planning without being detected," Mr Evans continued. "They probably had it all planned months ago and decided to pick their date.

"Everyone from Tony Blair down has said that it is timed for the G8 summit, but he doesn’t know that and neither does anyone else. If you want to make maximum impact, why not hit when everyone is in a state of jubilation after the Olympic win?"

Mr Evans said that a co-ordinated attack on this scale would have required significant expertise and a great deal of planning, and suggested that the bombers may have received training at al-Qaeda bases in Afghanistan. "This is unlikely to be the work of enthusiastic amateurs."

In March 2004, after the Madrid bombings killed 191 people, Ken Livingstone, the Mayor of London, warned that a terrorist attack in London was "inevitable".

Sir John Stevens, who was then commissioner of the Metropolitan Police, said that about 100 people trained by al-Qaeda were at large in Britain. "This is not just about the railways, the underground," he warned at the time. "It’s about buses, roads, pubs, nightclubs and the like."

A senior counter-terrorism official in the United States, who did not want to be identified, said that London was considered a prime target for Islamic extremists, in part because al-Qaeda was having difficulty getting people into the United States. He said that the attacks did not appear to be the work of suicide bombers.

After the attacks Charles Clarke, the Home Secretary, chaired meetings of the Government’s emergency response team, known as Cobra. Mr Clarke stood in for Tony Blair, who was travelling to London from the G8 summit in Gleneagles. A Cabinet Office spokesman was unable to provide details of the Cobra meeting.

The EU commissioner for justice and home affairs said that security forces throughout the continent would offer assistance. "It is necessary to immediately activate co-ordination between intelligence and police services and offer England all the help possible," he said.

The Department of Homeland Security has raised its colour-coded alert from yellow to orange, the second highest level, on public transport systems.

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