LONDON - MI5, the British intelligence agency, and the police are investigating "dozens and dozens" of al Qaeda inspired plots to cause mass murder in the UK, counter terrorism sources have disclosed.
The alleged plot to destroy up to ten transatlantic airliners, which police say they foiled last week, was just one of about 12 similar terrorist plans to kill hundreds of people currently being investigated.
Britain's Joint Terrorism and Analysis Centre today downgraded its security threat level from "critical" to "severe", saying intelligence suggested an attack was "highly likely" but not "imminent".
The Department for Transport has revised its security measures for airline passengers as a result of the downgrade. Click here for details.
Home Secretary John Reid said the threat level was downgraded because police believed the main suspects in the alleged plot had been arrested, but that there was still "a very serious threat of an attack".
He urged the public to remain vigilant.
Yesterday Mr Reid said "at least four" plots had been thwarted since last year's July 7 bomb attacks.
He also said that an al Qaeda operation in Birmingham had been foiled six years ago.
Asked about a report that "up to two dozen" terror investigations were underway, he told BBC News 24: "I'm not going to confirm an exact number but I wouldn't deny that that would indicate the number of major conspiracies that we are trying to look at.
"There would be more which are not at the centre of our considerations and there may be more we don't know about at all."
Detectives are continuing to question 23 British terrorist suspects arrested during raids in London, Buckinghamshire and Birmingham.
The alleged terror cell is accused of plotting to smuggle liquid explosive on US bound airliners and blow up nine or ten aircraft in three phases.
The alleged gang had close contacts with al Qaeda members in Pakistan, where several of the suspects are accused of travelling for training.
But police and intelligence chiefs have stressed that this plot is one of many they are currently investigating.
A senior counter terrorism source said: "Six weeks ago this [the alleged aircraft booming scheme] was one of a dozen plots that were being investigated. It was not our main priority. Things began to change and the inquiry was accelerated so that it became a priority.
"But it is by no means unique. There are dozens and dozens of plots like this one, in terms of people planning to commit mass murder in the UK.
"This is just the tip of the iceberg - people have got to start realising how serious the current situation has become."
MI5, the Security Service, are currently investigating about 1200 suspects in Britain who are considered to be actively engaged in terrorist related activities.
In the most recent raid, codenamed Operation Overt, counter terrorist officers believe that the alleged plot they foiled was largely organised and controlled by British Muslims living in this country, including allegedly one of the most senior al Qaeda figures operating in the UK.
Sources have confirmed that a Briton, named as Rashid Rauf, who was arrested in Pakistan last Wednesday, is an important member of the alleged cell and whose detention triggered the overnight raids in the UK.
However, Mr Rauf, who is thought to be the brother of Tayib Rauf, 22, one of those arrested in Birmingham last week, is not considered to be the leader of the alleged terror gang.
MI5 and the police are investigating whether there are any extremists who have been inspired or provoked by last weeks raids to strike in the next few weeks.
Checks are also taking place to ensure that any plotters, unknown to the authorities at the time, escaped.
Mr Reid will today chair of a meeting of Cobra, Whitehall's civil contingencies committee, to discuss the security situation.
Over the weekend he said: "This has been an ongoing threat, it is a chronic one and it is a severe one."
While the police and security forces were doing their job with 100 per cent effort, he said: "We can never guarantee 100 per cent success."
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UK investigating 'dozens of plots' for mass murder
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