The Al Qaeda may be planning to conduct new "Lockerbie-style" atrocities, and Britain could be among the frontline targets, experts have said.
Fears of a new Lockerbie-style bombing have increased after it emerged that one of the two bombs posted from Yemen was carried aboard two passenger planes before being seized in Dubai.
A bomb hidden in a suitcase on a Pan Am jet killed 270 people Dec 21, 1988, when it exploded over the Scottish village called Lockerbie.
US President Barack Obama's deputy national security adviser John Brennan said all nations had to fear the worst.
"We have to presume there are more mail bombs on the way. It would be very imprudent to assume there are no others," he was quoted as saying by The Mirror.
The explosive device was on a Qatar Airbus A320 from Yemen's capital Sanaa to Doha and from there it was transferred on to another Qatar Airways plane.
The second bomb was removed by bomb disposal experts at Britain's East Midlands airport.
The bomb was not due to explode for several more hours - when the aircraft would be over US cities.
A report said that devices, hidden in printer ink cartridges, were to be detonated by timers rather than mobile phones, which cannot receive a signal above 2,000 feet.
This means that instead of waiting until a plane is over the US, bombs could be timed to explode over European cities. And because it is a huge hub for international air freight, Britain faces the highest risk of attack.
One counter-terror official said: "It would appear Al Qaeda went for the jackpot this time, striking at America's heartland, and missed."
"The terrorists have tried repeated so-called dry runs. Next time they may lower their sights and simply detonate at the earliest opportunity. And that presents the spectre of Lockerbie on Britain all over again."