Det var britisk etterretning som avslørte at afghanske Najbullah Zazi (24) fra Denver, Colorado, deltok i planer om å sprenge t-banen i New York.
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.»
British spies help prevent al Qaeda-inspired attack on New York subway
British spies have foiled a terrorist plot by a suspected al Qaeda operative to blow up the New York subway.