Newly published memos disclose an escalating concern about the terrorist leader right up until the moment he launched his attacks on the World Trade Centre in New York and the Pentagon in Washington killing nearly 3,000 people.
But the files also reveal that MI5 discounted the threat from Islamic extremism just six years before the attacks and did not believe that America was vulnerable.
The author of the book, Defence of the Realm, has been given access to all of MI5s files dating back to its foundation in October 1909 in the first authorised history of an intelligence service anywhere in the world.
In a memo from July 6 2001, MI5 said the increase in the number of reports were sufficient to conclude that UBL [Osama bin Laden] and those that share his agenda are currently well advanced in operational planning for a number of major attacks on western interests.
Professor Christopher Andrew, the Cambridge University historian who wrote the book, added: Similar Security Service warnings to Whitehall of imminent attack continued at intervals over the next two months, up to and including the morning of 11 September. The intelligence received during the simmer of 2001, however, did not point either to a major attack in the United States or to an operation based on hijacked aircraft.
But MI5 had huge gaps in its knowledge about al-Qaeda and later on the morning of September 11, Sir Stephen Lander, then director general of MI5, told Tony Blair, the Prime Minister, that bin Laden was not head of a coherent unified terrorist structure according to a briefing note.
In an interview with the Daily Telegraph, Sir Stephen said MI5 had been slow to get going on the threat because they were pre-occupied with the IRA and that afterwards they had a major set to, sitting around a board room table working out how to move on.
MI5 had opened a permanent file on bin Laden in September 1995, but they believed he was a terrorist financier rather than a leader, adding the source of his wealth was a mystery.
In December 1995, a memo to the heads of police Special Branches, said: Suggestions in the press of a world-wide Islamic extremist network poised to launch terrorist attacks against the West are greatly exaggerated The contact between Islamic extremists in various countries appears to be largely opportunistic at present and seems unlikely to result in the emergence of a potent trans-national force.
When Dame Stella Rimington visited the US in March 1996, a few weeks before her retirement as head of MI5, she had never heard the name al-Qaeda, the book reveals.
Nevertheless MI5 scored an intelligence triumph shortly afterwards when they became the first western intelligence agency to gain a recording of bin Ladens voice.
But a memo from 1999 admitted: The allied intelligence community does not have a clear view of UBLs terrorist planning process. Even the most reliably sourced intelligence on this question usually consists of a snapshot of a proposed plan being discussed. Most of the reporting does not make clear how advanced the plan is. It is rare that intelligence has named those who are to take part ion a planned attack.
Later in the summer of 1999, MI5 reported: Intelligence suggests that while UBL is seeking to launch an attack inside the US, he is aware that the US will provide a tough operating environment for his organisation.
Unknown to MI5, Britain had already been targeted by al-Qaeda. It was only after September 11 that officers discovered that a Pakistani microbiologist called Rauf Ahmad, also known as Abdur Rauf, who traveled to a conference in Britain in September 2000 to try and buy pathogens from fellow delegates, was a high-ranking member of al-Qaeda. (10.05.2009, Duncan Gardham, Security Correspondent) http://www.telegraph.co.uk/news/uknews/terrorism-in-the-uk/6262491/MI5-warned-that-bin-Laden-was-planning-attacks-on-morning-of-911.html