The Joint Terrorism Analysis Centre, which operates from the headquarters of MI5, had uncovered no evidence in recent weeks of an al-Qaeda-style plot to attack London.

The terrorist alert level had been lowered by one grading more than a month ago because it had been judged that the threat, while still high, was lower than it had been since the attacks in the United States on September 11, 2001.

MI5, which advises the Government on appropriate terrorist alerts, had recommended that the threat status could safely be reduced from “severe general” to “substantial”.

Before the attacks yesterday, Sir Ian Blair, the Metropolitan Police Commissioner, spoke on the BBC Radio 4 Today programme at 7.20am. He said: “We have been described by Her Majesty’s Inspectorate of Constabulary as the envy of the policing world in relation to counter-terrorism and I am absolutely positive that our ability is there.”

On the possibility of an attack, he added: “It is difficult to calculate whether it is inevitable that they will get through.”

Since the Bali bomb in October 2002, which killed 202 people, the Government has introduced a more complex series of threat levels because it was felt that the old system was not sufficiently flexible. There are now seven gradings, and “substantial” is fourth, behind “severe general”, “severe specific”, where there is definite intelligence, and “imminent”.

Security sources said there had been no reason to raise the alert status because it was high enough, and there had been no indications of any plot to hit London to coincide with the G8 summit in Scotland or the announcement of the host city for the 2012 Olympic Games.

The sources said it was realised that there would be accusations that the intelligence services had failed. “But we have always warned that intelligence never reveals a complete picture. It’s a cheap shot to blame the intelligence services, but nothing was known in advance of these attacks,” an official said.

One key aspect for the security and intelligence services is to try to pinpoint the precise motivation behind the attacks. Tony Blair said in his statement that the terrorists had chosen the G8 summit to launch the bombings.

However, security experts said that after the jubilation over the announcement of the Olympics for London it would be a classic strategy for alQaeda to erase that jubilation with a shock attack. “Jubilation one day and disaster the next, that’s the sort of impact these terrorists like to make,” one intelligence expert said.

The terrorist threat level had been maintained at “severe general” — the third highest — throughout the general election because of the perceived risk of a possible attack to disrupt voting. It had also been at that level for the wedding of the Prince of Wales in April.

Unlike some European countries, including France, the high threat level had remained unchanged since the September 11 attacks, because of the known ambition of al-Qaeda to hit Britain, as America’s strongest European ally.

However, the G8 summit was not considered to be an event that by itself merited raising the alert level. This would have happened only if there had been a surge in intelligence indicating that al-Qaeda or an affiliated group had begun to focus on the summit.

Reports that Israel had given notice of possible terrorist attacks in London were dismissed by the Israeli Embassy and by British security officials.

Intelligence officials said that the terrorism analysis centre would re-examine all recent secret material to see whether, with hindsight, there had been indications that might have been missed.

The centre, which has the prime responsibility for assessing intelligence on international terrorism and answers to Dame Eliza Manningham-Buller, the Director-General of MI5, had been in close contact with other European agencies because of the threat to European capitals after the Madrid bombing in March last year.

There is no indication as yet that yesterday’s bombings were directly linked to the Madrid atrocities. British officials said that there were similarities in the co-ordinated attacks and indicated that some of the explosives may have been detonated by remote control, using mobile phones — again, similar to methods used in Madrid.

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