It is thought the plan was to detonate explosive devices smuggled in hand luggage on to as many as 10 aircraft.
Police are searching premises after 21 people were arrested. Home Secretary John Reid said they believed the “main players” were accounted for.
High security is causing delays at all UK airports.
The threat level to the UK has been raised by MI5 to critical after the arrests in London, High Wycombe and Birmingham.
Critical threat level – the highest – means “an attack is expected imminently and indicates an extremely high level of threat to the UK”.
Three US airlines are believed to have been targeted.
Mr Reid said had the attack gone ahead it would have caused a loss of life of “unprecedented scale”.
He said they were “confident” the main players were in custody, but neither the police nor government are “in any way complacent”. [Read on]
Also on: FOX News