Shutting down the Bin Laden unit squandered 10 years of expertise in the war on terror, said Michael Scheuer, who founded the unit in 1995 and arguably knows more about Bin Laden than any other western intelligence official. He believes the unit was dismantled because of bureaucratic jealousies within the CIA, and that the closure delivers a further setback to a pursuit that has been squeezed for resources for the past two years.

"What it robs you of is a critical mass of officers who have been working on this together for a decade," he told the Guardian. "We had a breed of specialists rare in an intelligence community that prides itself on generalists. It provided a base from which to build a cadre of people specialising in attacking Sunni extremist operations, who sacrificed promotions and other emoluments in their employment in the clandestine service, where specialists were looked on as nerds."

A 22-year CIA veteran, Mr Scheuer became aware of Bin Laden in the 1980s when the Saudi-born militant was on the fringes of the US-backed mujahideen fighting the Soviet occupation of Afghanistan. In 1995, when western intelligence agencies knew little about Bin Laden, Mr Scheuer was charged with setting up a unit that would track what support he was giving to Islamist groups, and determine whether he was a threat. Mr Scheuer left the agency in 2004 after writing a scathing book about its counter-terrorism efforts, called Imperial Hubris. He is now a consultant on terrorism.

CIA officials disclosed this week that the Alec unit - named after Mr Scheuer's now teenage son - had been disbanded, and its agents reassigned. The agency described the shakeup as a necessary adaptation to the changing nature of the US war on terror. "The reorganisation just reflects the understanding that the Islamic jihadist movement continues to diversify," a US intelligence official said.

Mr Scheuer said he disagreed with the argument it was making that Bin Laden was isolated, the organisation was broken and that he was now just a symbol. "How do you explain the fact that he is able to dominate international media whenever he wants to?"

Yesterday, the FBI said it had foiled a plot to blow up the railway tunnels which run beneath the Hudson river into Manhattan, and send a torrent of water gushing towards Wall Street. The leader of the cell, Assem Hammoud, 31, amember of al-Qaida, and two other plotters were arrested in Beirut. None of the conspirators had ever entered the US.

In video footage released on the internet on Thursday and yesterday, Bin Laden's deputy, Ayman al-Zawahiri, claimed to have directly masterminded last year's London bombings. He said two of the bombers, Mohammad Sidique Khan and Shehzad Tanweer, had received explosives training at al-Qaida camps, and the organisation had helped to choose the targets. In recent weeks authorities have made arrests of alleged al-Qaida activists in Manchester, London, Toronto and Miami.

"Bin Laden has always said the main activity of al-Qaida is the instigation ... of Muslims to jihad," Mr Scheuer said.

"All of the people who have been picked up have said they were inspired by Bin Laden, that they trained in their own countries and used information picked up on the internet.

"So the fire that Bin Laden was trying to set is what we are beginning to see around the world and, unfortunately, nowhere more than in the west."

He said the dismantling of the unit reflected a myopia in an intelligence community uncomfortable with the independence of the CIA agents who championed Bin Laden's pursuit.

"From the very beginning, Alec was an anomaly in that it was not subordinated to any area division, and it was given the authority to communicate with overseas stations - with or without the permission of area divisions. That caused a great deal of heartburn among very senior leaders at the agency," he said.

The infighting went on for years. In 1998, a few months before car bombs blew up US embassies in Tanzania and Kenya, Mr Scheuer was told to be ready to disband - only to see the unit saved by the then agency director, George Tenet.

But it was only a temporary reprieve. The staff was reduced from 25 to eight. More recently, the unit became a repository for inexperienced officers, who were rotated in for 60- or 90-day stints.

During the past two years the hunt for Bin Laden came second to fighting the insurgency in Iraq. With the worsening security situation in Afghanistan, more intelligence resources are being diverted towards propping up the government of Hamid Karzai in Kabul than to tracking down leads on Bin Laden.

The Bush administration continues to claim success in the fight against al-Qaida, noting that it has killed off two-thirds of its known leadership at the time of the September 11 2001 terror attacks.

But as Mr Scheuer notes, al-Qaida seems to have no difficulty in replacing its senior leaders. A decade after he first began keeping tabs on Bin Laden, he continues to be surprised by al-Qaida's resilience.

"I don't think anyone could have expected to see the success of such an organisation," he said. For observers familiar with the disunity of the Palestinians and other Arab causes, al-Qaida was a departure from the known.

"One of the things that really slowed down the western response to Sunni extremists, but al-Qaida specifically, is that when intelligence agents looked at a group made up of Yemenis, Egyptians, Saudis, Algerians and converts, the natural response was to say, 'That is not going to last 10 minutes. They can't get along together.' It took a long time for people to realise we were seeing an animal of a very unique nature. We haven't even begun to understand where our enemy is coming from."