Scotland Yard alerted the FBI to the biggest terror plot against America since 9/11 after intercepting an email.
It sparked 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 said to have been part of a gang that used stolen credit cards to buy bomb ingredients 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.
Thousands of New Yorkers could have been at risk from a US subway blitz. Zazi, of Denver, Colorado, is alleged to have been given instructions through the internet by a senior al-Qaeda figure in Pakistan.
But the Yard's counter-terrorism branch was monitoring an email address uncovered in April's abortive Operation Pathway probe into an alleged UK terror cell.
Eleven Pakistani suspects were arrested prematurely and later freed after Met anti-terror chief Bob Quick was pictured with a secret document on show while going to brief the PM.
Quick resigned over the gaffe, but officers still kept tabs on the email address.
It lay dormant for months - then was suddenly reactivated in September.
After a British tip-off, US authorities allegedly found bomb-making instructions on Zazi's laptop. His fingerprints were on batteries and measuring scales were seized.
A phone contained video of New York's Grand Central Station, which he visited a week before his arrest. And explosive residue was found in a kitchen. Informants also said Zazi visited a terror training camp in Pakistan. A British security source told The Sun: "This was excellent work and highlights the fact we produce good information."
Furious US intelligence bosses had threatened to stop sharing secrets with Britain after the dying Lockerbie bomber was freed in August.
But the source said: "They were delighted with the intelligence we gave them and believe it helped prevent a catastrophic attack."
Source: The Sun