SECURITY sources believe that a man arrested in last week's anti- terror raids in Britain is Al-Qaeda's leader in this country.
Home Office officials say that one of those arrested is suspected not only of masterminding the foiled plot to bring down up to nine transatlantic airliners, but also of involvement in other planned atrocities over the past few years.
They believe that he was instrumental in sending the ringleader of at least one previous British terror plot for training at a camp in Pakistan last year. He is described by counter-terrorist officials at MI5 as the senior figure in a British terror network involving Kashmiri, north African and Iraqi cells.
Scotland Yard believes that the plan to bring down airliners involved up to 20 terrorists who were planning to smuggle liquid bomb components in hand luggage onto nine British and American passenger jets.
Their targets were planes leaving Heathrow and possibly three other British airports later this month.
The bombs were to be assembled on board by combining peroxide and acid-based substances into liquid explosives. The plan was to explode the devices simultaneously as the planes headed for cities in America.
Paul Stephenson, deputy commissioner of the Metropolitan police, said it was a plot "to commit mass murder on an unimaginable scale". It is estimated that as many as 3,000 people could have been killed.
The thwarting of the alleged plot has, however, failed to quash continuing fears among counter-terrorist experts. Senior security officials have briefed ministers that a "second phase" of attacks may be about to be launched.
At least two suspects escaped last Wednesday night's police raids. Although they are not thought to be significant players, there remain concerns that they may now be galvanised into taking some form of unspecified action.
Ayman al-Zawahiri, Al-Qaeda's deputy head, is said to have warned in a message placed on a restricted extremists' website last month that the terrorist group was planning two large-scale attacks this autumn.
The FBI has assigned 200 agents to follow up any leads that come out of the British investigation. Security sources said that separate surveillance operations by police and MI5 were continuing into a variety of suspected plots by other terror cells.
These included plans, said to be in their early stages, to target ferry ports, the railway network and the London Underground. Police say they are also stepping up patrols at some mainline railway stations.
A senior transport security official said: "The question is: have we got everybody? If they are going to find airports too difficult, the railways aren't a bad second choice." However, he emphasised that there was no specific intelligence that the railways were under imminent threat.
Police sources claimed yesterday to have seized "high grade evidence" including chemicals, documents and a video during last week's raids in east London, Birmingham and High Wycombe in Buckinghamshire.
They believe they have arrested "the ringleaders, the technical experts and the foot soldiers" behind the plot. "The leadership was very professional," said a police source.
The investigation into the suspected Al-Qaeda leader in Britain and his UK associates was considered by Eliza Manningham-Buller, MI5's director-general, to be the security service's single most important line of inquiry.
He is suspected of being behind two "pipelines" which saw potential terrorist recruits being sent for training at camps in Pakistan and to join the "holy war" in Iraq.
The Al-Qaeda leader — who cannot be named for legal reasons— acts as a suspected hub in a network of extremist groups. These include Kashmiri and north African groups based in this country. He is linked to a second suspect also in Britain who has "played a major role in facilitating support for the Iraq jihad".
A third associate is an Iraqi who came to Britain in 2004 and worked on providing support for British extremists who wanted to travel to Iraq to fight the "holy war".
MI5 said he acquired weapons in preparation for an unspecified attack in Britain. He was detained in January last year pending deportation to Iraq.
The British leader's suspected links with other Al-Qaeda figures in Pakistan have been the subject of intense MI5 scrutiny since last August.
It was the arrest of another associate in Pakistan last week that prematurely triggered Operation Overt, the counter-terrorist plan that is said to have foiled the transatlantic airliner plot.
Contrary to claims by the Pakistani government, the arrest was not anticipated in London. There were also conflicting reports about the reasons for the suspect's arrest.
One Pakistani official said he had been under surveillance for several weeks following a tip-off from Britain. He was said to have been monitored visiting radical imams and seminaries that had been linked to terrorism. The official said he had travelled to some of the same places as Mohammad Sidique Khan and Shehzad Tanweer, two of the July 7 bombers.
Some of those arrested were reported to have travelled to Pakistan to engage in charity work following last October's earthquake.
John Reid, the home secretary, reviewed the security situation when he chaired a further meeting of Cobra, the emergency committee which is co-ordinating the government's response.
Although police say that they have arrested all those they wanted, the country still remains on "critical" alert. This means that MI5 believes that a further attack could be imminent.
The Metropolitan police said 23 people remained in custody last night. One person was released on Friday.
The origins of Operation Overt are said by some American officials to have begun in 2003. However, the full-scale inquiry is not thought to have been launched until last August when dozens of surveillance officers were assigned to monitor the British Al-Qaeda leader's home.
A Pakistani official close to the intelligence services there was reported yesterday as saying that there may have been a British mole planted by the security services inside the terror cells in the UK.
Two of those arrested last week are said to have visited Pakistan in the months before the planned attacks. They are said to have met Matuir Rehman, an Al-Qaeda suspect and specialist in explosives.
After the two Britons returned to this country, they are believed to have received a wire transfer of money.
Pakistani authorities say the man arrested there last week had fled the West Midlands several years ago. He had received training in explosives at Al-Qaeda camps along the Afghan-Pakistan border and had organised Al-Qaeda funding for the organisation's leader in Britain.
One agent said he had been under surveillance for several weeks following a tip-off from British intelligence which had been following up several Pakistan-related leads from its investigation into last year's July 7 bombings.
It was British detectives who uncovered the role of the man arrested in Pakistan last week and tipped off their Pakistani counterparts.