The top terrorist – who had a £4.25million bounty on his head – was said by US sources to be finalising attacks on Western targets.
He is also thought to have been the brains behind toothpaste bombs and clothes impregnated with explosives designed to blow up airliners in mid-flight.
Khorasan is understood to have been testing sophisticated bombs at its Syrian HQ so they could evade airport detectors.
Yesterday a US official said it was believed Al Fadhli was dead.
The importance of his death can be seen from comments made earlier this month by James Clapper, the US Director of National Intelligence, who said: “In terms of threat to the homeland, Khorasan may pose as much of a danger as the Islamic State.”
Khorasan was in Syria trying to recruit Western jihadists to send them back to their home countries on suicide missions.
Al Fadhli, 33, was born in Kuwait and by the age of 20 was sufficiently high-ranking in Al Qaeda to have known about the 9/11 attacks in advance.
US Army Lieutenant General William Mayville Jr, director of operations for the Joint Chiefs of Staff, said: “We’ve been watching this group closely for some time, and we believe the Khorasan group was nearing the execution phase of an attack either in Europe, or the homeland.”
A ban on uncharged mobile phones and laptops this summer on air passengers was introduced due to the threat posed by Khorasan and Western jihadists.