The '20th hijacker' had been a suspect for years - but he was ignored by intelligence agencies

British and American intelligence agents trying to destroy the al-Qa'ida network worldwide have been forced to reassess the role of a London-based French Islamic radical who, according to the latest evidence, could have led them straight to the heart of the suicide hijacking conspiracy.

The man in question, Zacarias Moussaoui, was arrested in Minnesota on an immigration violation – nearly three weeks before the attacks on America – after raising suspicions at a local flying school. Previously, he lived in south London on and off for nine years, where he was a follower of the radical Islamic cleric Abu Qatada, recently named as the head of Osama bin Laden's network in Europe.

Mr Moussaoui's case has been a source of official embarrassment from the start. After slipping through the hands of British intelligence, he was never properly investigated by the Federal Bureau of Investigation despite repeated warnings from French intelligence that he was a member of al-Qa'ida.

It has now emerged that Mr Moussaoui made numerous telephone calls to known associates of the 11 September hijack gang who were then living in Hamburg and received $15,000 (£10,500) in bank transfers from them shortly before setting off to Minnesota for flight simulation training.

That would suggest that he was himself earmarked to be one of the hijackers. French investigators believe he was asked to replace Ramsi Bin Al-Shibh, a Yemeni citizen who lived with Mohamed Atta and some of the other hijackers in Hamburg but failed on three occasions to obtain a US visa.

The FBI is convinced there was supposed to have been five hijackers on each of the four planes seized and crashed on 11 September. On one of them, United Airlines Flight 93 from Newark to San Francisco, there were just four – leading investigators to presume the existence of a "20th man" who never made it.

The FBI says Mr Bin Al-Shibh, now on the run, was that 20th man. But the French information suggests that Mr Moussaoui was Mr Bin Al-Shibh's understudy.

Mr Moussaoui, a French citizen of Moroccan origin, raised suspicions at the Pan Am International flight school in Eagan, Minnesota, because he wanted to learn how to fly a passenger jet at cruising altitude, but not how to take off or land. He was jailed for visa irregularities but not considered worthy of investigation under the Foreign Intelligence Surveillance Act despite the French warnings.

FBI agents decided they did not have enough evidence to argue before a judge that he posed an imminent threat. Had agents searched his computer drive, as they later did, they would have found copious information on crop-dusting planes – a possible method for disseminating biological or chemical agents. Had they followed up on his phone records, they would have found evidence of conversations with Mr Bin Al-Shibh and also with Mr Atta's landlord at the Hamburg flat.

That information, along with the money transfers, might have been enough to expose the Hamburg cell, which investigators believe was the key planning unit for 11 September. The $15,000 appears to part of a war chest of more than $200,000 wired to the hijacking team in the weeks leading up to the attack, most of it sent from an account in the United Arab Emirates, according to US investigators.

The missed opportunities go back further, to the time Mr Moussaoui spent in Britain, starting in 1992. As early as 1994, a French investigating magistrate, Roger Leloire, was in London digging up leads on the assassination of three French consular officials in Algeria and trying to find a match for an individual identified only as "Zacarias". Mr Moussaoui was doing a masters degree in business at South Bank University at the time.

In 1999, French intelligence learned that Mr Moussaoui had gone to Afghanistan and was suspected of having attended one of Mr bin Laden's training camps. The French warned their British counterparts, who appear to have done nothing with the information. A senior intelligence source denied last night that Britain's spy agencies were ever told by France that Mr Moussaoui was a suspect in a specific case. He said Mr Moussaoui's name had been mentioned in routine information traffic between Paris and London, but no specific requests had ever been made by the French.

The Spanish authorities have also released details of phone conversations between the alleged head of an al-Qa'ida cell in Madrid, Imad Eddin Barakat Yarkas, and an interlocutor in London identified only as "Shakur".

Since 11 September, US officials have sought to minimise Mr Moussaoui's role. They are holding him as a material witness, but have yet to file charges – in part because he has refused to co-operate with their investigation. Some officials have also told US newspapers they do not believe he played more than a marginal part in the 11 September plot.

The French, meanwhile, have leaked their own information to the media to play up the fact that they were on to Mr Moussaoui but that their warnings were ignored. French intelligence informed the Americans about his al-Qa'ida links on 1 September, and again in a bilateral meeting of intelligence agents in Paris on 5-6 September. According to an account of that meeting in Le Monde, US participants said Mr Moussaoui's case was in the hands of the immigration authorities and was not a matter for the FBI.

MI5 has recently held a series of meetings with officials from South Bank University in south London, discussing Mr Moussaoui, who studied there for two years. A spokesman for South Bank University said: "There has been a series of discussions which have gone on relating to Mr Moussaoui concerning the security services. There have been three subsequent meetings relating to a series of matters relating to 11 September, with which we have been co-operating with the authorities."

The French intelligence service, DST, took an increasing interest in the Londoner and during 1999 he was observed making trips to Pakistan and Afghanistan. French investigators claim MI5 was alerted and asked for Mr Moussaoui to be placed under surveillance. The request appears to have been ignored.

After arriving in Britain in 1992, he become attracted to events at the Fourth Feathers Centre, where an Islamic cleric, Abu Qatada addressed an eager audience of young radicals. Others attending the meetings included Djamel Beghal, a 36-year-old Algerian who moved to London from France in 1997 and was arrested in Dubai in July this year for allegedly being part of a plot to blow up the American embassy in Paris.

Mr Moussaoui lived on the top floor of a housing association block in Streatham and later in a ground-floor flat in Brixton, with a north African girlfriend who has been sought by police since 11 September.

Neighbours remember Mr Moussaoui as speaking good English and being "well-dressed and intelligent".

A similar favourable impression was gained by Colin Knapp, Mr Moussaoui's course director at South Bank, from where he graduated in 1995. Mr Knapp said Mr Moussaoui did not express political views and chose to wear Western clothing.

Mr Moussaoui's family had noticed something amiss. His brother, Abd-Samad Moussaoui, said: "He began to change when he went to Britain. It was there that he got drawn into an extremist group."

In America Mr Moussaoui behaved suspiciously from the start. He would not divulge his real name, but went by the pseudonym Zuluman Tango Tango. He did not obtain his licence and abandoned his course in May, Then, in August, things changed after telephone conversations between Mr Moussaoui and the Hamburg apartment where Atta lived with other associates linked to the 11 September atrocities.

After his arrest he was found with a French passport, with an outdated American visa obtained in Islamabad, and a fake Algerian passport. Nothing was done until after the attacks, when Mr Moussaoui was seen cheering as he watched television pictures of the destruction from his secure unit. The Minneapolis FBI then checked his computer and found information on crop-spraying from the air, prompting fears that chemical and biological attacks were being prepared. Mr Al-Attas was rearrested and pumped for more information. Neither he nor Mr Moussaoui seems to have been willing to talk.