By Stefano Ambrogi
LONDON (Reuters) - A bomb found on a U.S.-bound cargo plane was designed to go off on board the aircraft and could have been powerful enough to bring it down, possibly over Britain, ministers said on Saturday.
Interior minister Theresa May said the device, found at a British airport on Friday, was a working bomb that could have exploded.
"The target may have been an aircraft and had it detonated the aircraft could have been brought down," May said after a meeting of a government crisis committee.
Prime Minister David Cameron said authorities believed the device was "designed to go off" aboard the plane. He said he could not rule out the intention may have been to explode the bomb on British soil.
The government said it would immediately stop the movement of air freight from Yemen into or through Britain. That included air cargo brought in on indirect flights from the country.
Two U.S.-bound packages from Yemen containing explosive material were intercepted in Britain and Dubai on Friday, in what President Barack Obama called a "credible terrorist threat."
The device found in Britain was taken off a cargo plane at East Midlands airport north of London. British police said it was trans-shipped at Cologne Bonn airport in northwest Germany.
Direct cargo and passenger flights from Yemen to Britain were suspended in January, following an attempt to bomb an aircraft destined for Detroit.
British media described the bomb as a print-toner cartridge, covered with white powder, with wires protruding from it.
Aviation expert Chris Yates told BBC Radio his sources had told him the device was very carefully concealed with a "mobile phone attached to it," which might have acted as a detonator.
Security experts have linked the bombs to al Qaeda.
SPY CHIEF WARNING
Only on Thursday Britain's top spy, MI6 chief John Sawers, in the first public speech by a serving UK espionage boss, said terrorism might hit the West again "at huge human cost."
Britain has regularly been a focus of Islamist militant plots. In the worst attack in Britain, suicide bombers killed 52 people on the London transport network on July 7, 2005.
The latest incident brought back memories of August 2006 when British police said they had thwarted a plot to blow up several U.S.-bound airliners over the Atlantic using liquid explosives.
That plot caused chaos at British airports as authorities imposed tough restrictions on carry-on luggage.
This time, there was little public alarm and no sign of increased security at airports which were operating normally.
Philip Baum, editor of Aviation Security International, told Reuters freight security had long been regarded as vulnerable.
"From an aviation point of view it demonstrates the limitations of the existing system vis-a-vis cargo security," he said.
(Additional reporting by Adrian Croft)