Prime Minister David Cameron says the device in a package sent from Yemen and found on a US-bound cargo plane was designed to go off on the aircraft.
But Mr Cameron said investigators could not yet be certain about when the device, intercepted at East Midlands Airport, was supposed to explode.
A second device containing explosives was found on a cargo plane in Dubai. The US suspects al-Qaeda involvement.
In Yemen, police have arrested a woman suspected of posting the packages.
She was detained in the capital, Sanaa, after being traced through a telephone number she had left with a cargo company, officials said.
The unnamed young woman, described as a medical student and the daughter of a petroleum engineer, was arrested on the outskirts of the city, a security official told AFP news agency. Her mother was also detained but was not a prime suspect, the arrested woman’s lawyer said.
President Ali Abdullah Saleh said the US and the United Arab Emirates had provided Yemen with information that helped identify the woman and pledged that his country would continue fighting al-Qaeda “in co-operation with its partners”.
“But we do not want anyone to interfere in Yemeni affairs by hunting down al-Qaeda,” he added, as heavily armed troops patrolled Sanaa….