The plan, which reportedly would have been the biggest attack on America since 9/11, was uncovered after Scotland Yard intercepted an email.

The force alerted the FBI, who launched an operation which led to airport shuttle bus driver Najibullah Zazi, 24, being charged with conspiracy to use weapons of mass destruction.

The Afghan is alleged to have been part of a group who used stolen credit cards to buy components for bombs including nail varnish remover.

The chemicals bought were similar to those used to make the 2005 London Tube and bus explosives which killed 52 people.

Zazi, from Denver, Colorado, is understood to have been given instructions by a senior member of al Qaeda in Pakistan over the internet.

US authorities allegedly found bomb-making instructions on his laptop and his fingerprints on batteries and measuring scales they seized.

A phone containing footage of New York's Grand Central Station, thought to have been made by him during a visit a week before his arrest, was also found along with explosive residue. Zazi was also said by informants to have attended a terrorist training camp in Pakistan.

The alleged plot was unmasked after an email address that was being monitored as part of the abortive Operation Pathway was suddenly reactivated.

Operation Pathway was investigating an alleged UK terrorist cell but went awry after the then Met Police counter-terrorism head Bob Quick was pictured walking into Downing Street displaying top secret documents.

Eleven Pakistani suspects were arrested immediately after the gaffe but later released without charge.

However, security staff continued to monitor the email address which eventually yielded results.

The British discovery also came at just the right time – the US had threatened to sever intelligence links over the release of Lockerbie bomber Al Megrahi.

A British security source told The Sun: "This was excellent work and highlights the fact we produce good information.

"(The US authorities) were delighted with the intelligence we gave them and believe it helped prevent a catastrophic attack."