Bomb plot 'just the tip of the iceberg' of UK al-Qa'ida activity

MI5 and the police are investigating "dozens and dozens" of possible al-Qa'ida-inspired plots to cause mass murder in Britain, counter-terrorism sources have disclosed.

The alleged plot to destroy up to 10 transatlantic airliners, which police say they foiled last week, was believed to be just one of about 12 similar terrorist plans that are currently being investigated.

Discussing the threat, the Home Secretary, John Reid, defended his decision to leave the nation's terror alert level at "critical", although late last night the Government announced that it had been downgraded to "severe", meaning that the threat of attack was still "highly likely", but not "imminent".

Earlier, Mr Reid said that "at least four" plots had been thwarted since last year's July 7 attacks in London. He also said that a suspected al-Qa'ida operation in Birmingham had been foiled six years ago.

Asked about a report that "up to two dozen" terror investigations were under way, 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."

Announcing the downgrade in the security threat level, Mr Reid said it had been decided on the basis that the "main suspects" in the alleged plot were arrested last Thursday. But he stressed there was still a " very serious threat of an attack" and said the public should remain vigilant.

Detectives are continuing to question the 23 British terrorist suspects arrested in east London, Buckinghamshire and Birmingham. The alleged terror cell is accused of plotting to smuggle liquid explosives on US-bound airliners and blow up nine or 10 aircraft in three phases and is suspected of having close contacts with al-Qa'ida in Pakistan.

But police and intelligence chiefs stressed that this plot was only one of many they were investigating. A senior counter-terrorism source said: " Six weeks ago, this [the alleged aircraft bombing 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 is currently investigating about 1,200 suspects in Britain 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-Qa'ida figures operating in the UK.

Sources have confirmed that a Briton, named as Rashid Rauf, who was arrested in Pakistan last Wednesday and whose detention triggered the overnight raids in the UK, is suspected of being an important member of the alleged cell.

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 gang.