Intelligence about a cell with access to explosives and plans to unleash a “third wave” of attacks was the trigger for last Thursday’s unprecedented security exercise. The operation saw 6,000 police, many armed, patrolling across London.

Senior police officers say that there was “specific” intelligence from several sources that an attack was planned for that day. The disclosure contradicts official statements by Scotland Yard that Thursday’s security exercise — the biggest since the second world war — was simply a precaution aimed at reassuring the public.

The disclosures come as a suspected bomber detained in Italy apparently admitted to involvement in the attacks on July 21. According to Italian reports, Hussain Osman has alleged to investigators that the leader of the July 21 attacks was Muktar Said-Ibrahim, who was detained in London on Friday.

Osman claimed Ibrahim, the alleged bus bomber, had taught him how to make bombs. But he also claimed the incidents on July 21 were intended to be a political statement rather than to take lives.

Details of a “third wave” terror plot to carry out multiple suicide attacks were disclosed to senior police commanders at an emergency Special Branch conference held at Scotland Yard last Wednesday. All police leave was cancelled and hundreds of officers were instructed to book into central London hotel rooms.

Members of the third cell are said to be independent of the July 7 and July 21 terrorists but have “associations” with some of the suspects who have been arrested in connection with the July 21 attacks. Deputy Assistant Commissioner Peter Clarke, head of the anti-terrorist branch, said that despite capturing the four suspected bombers and a fifth man linked to the cell “the threat remains and is very real”.

Another officer, a member of the Yard’s firearms unit, which captured three of the suspected suicide bombers in two raids in west London, said: “What we did on Friday was just the tip of the iceberg. There is some big stuff coming in the next few months. There’s a big network that’s got to be cracked.”

Osman, a 27-year-old asylum seeker from Ethiopia who has British citizenship, was arrested by Italian police at his brother’s flat in Rome after an international manhunt.

He is reported to have travelled to France via the Eurostar and then to Italy. Shortly before his arrest, Osman made one phone call to a Saudi Arabian mobile number. Osman is also said to have confessed almost immediately to Italian police. “Yes, it is true, I was there on July 21. I’d been given a rucksack,” he reportedly told police.

Osman is said to have claimed the attacks had been planned by Ibrahim after the two had met at a gym in Notting Hill, west London. He said they had acted independently, had no links to the July 7 attacks, in which 56 people died, and had been taken by surprise by the suicide bombings two weeks earlier.

His group decided to carry out the attacks as a statement about the war in Iraq but was not linked to Al-Qaeda or any other terrorists. Contrary to some reports, he told his interrogators that the plotters did intend to explode their rucksacks but that they did not intend to kill anybody.

He is reported to have said: “Religion had nothing to do with this. We watched films. We were shown videos with images of the war in Iraq. We were told we must do something big. That’s why we met.”

Osman, who is suspected of the Shepherd’s Bush attack, claimed they had not meant to kill anyone. “I didn’t want to kill, ours was supposed to be a demonstrative act,” he is said to have told interrogators. “We planned to carry out an attack. We didn’t want to kill, only to spread terror.”

Osman appeared at a hearing yesterday where Italian magistrates received a British government request for his urgent extradition. He objected to extradition. His lawyer said it could take up to two months for him to be returned to London.

Ibrahim is being questioned at Paddington Green top security police station in London. Yasin Omar, the suspected Warren Street Tube bomber, was arrested last week. The fourth man, Ramzi Mohammed, the suspected Oval Tube bomber, was arrested with Ibrahim. Ramzi’s brother, Wahbi, 22, is being questioned about the discovery of a discarded fifth bomb.

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