UK intelligence services were warned of the threat posed by al-Qaeda in the
mid-90s but did not act quickly enough, says a spy who infiltrated the network.
Hamza was spied on by Nasiri
The agent told the BBC he warned his handlers that al-Qaeda was better organised than was previously thought.
Known by the pseudonym of Omar Nasiri, he worked for intelligence in the UK and France, and trained in Afghanistan.
Mr Nasiri also claimed a senior al-Qaeda operative planted evidence to provoke the US into war against Iraq.
Ibn Sheikh Al-Libi fabricated links between al-Qaeda and Iraq when being questioned by US forces in late 2001, he said.
BBC security correspondent Gordon Corera said the revelations had provided an "insight into the emergence of al-Qaeda in the 90s".
He said French authorities had been concerned about Algerian terrorists and British authorities feared attacks in the UK.
But the details given by the agent, who worked for both the French and British intelligence services, showed that "people were not necessarily looking at those global interconnections".
He added that the "legacy" of the Afghan training camps of the 1990s was being seen in the form of global attacks.
Mr Nasiri trained at al-Qaeda's camps in Afghanistan and met senior figures from the group.
He described the use of chemical weapon experiments and claimed radical Muslim cleric Abu Hamza al-Masri was given terror training in London.
The spy also said that to maintain his cover the French and UK services reluctantly gave him money to pass to al-Qaeda, and he did not know how the money was spent.
When asked if he ever forgot he was a spy while in Afghanistan, "Omar" replied: "Oh yeah, all the time."
Al-Qaeda first came to prominence after the attacks on 11 September 2001 in the US.