The bomb found on the cargo plane at East Midlands Airport last month was timed to detonate over the eastern seaboard of the United States, Scotland Yard said today.
The device, which was removed from a UPS aircraft by Leicestershire police officers shortly after 3.30am on October 29, was timed to go off some seven hours later, police said.
A Scotland Yard spokesman said: "If the device had not been removed from the aircraft the activation could have occurred over the eastern seaboard of the US."
The bomb, disguised as an ink cartridge, was found following a tip-off from Saudi intelligence.
A Scotland Yard spokesman said the cargo plane arrived at East Midlands Airport from Cologne at 2.13am BST and left at 4.20am BST "after the suspect package had been removed".
"Forensic examination has indicated that if the device had activated it would have been at 10.30am BST," he said.
"If the device had not been removed from the aircraft the activation could have occurred over the eastern seaboard of the US."
He added that the bomb was "disrupted" when explosive officers removed the printer cartridge from the printer during their initial examination of the device at about 7.40am BST.
Experts in Germany said the bomb, and another found in Dubai, contained at least 300g (10.58oz) of the powerful explosive PETN.
It had travelled through a UPS hub at Germany's Cologne airport before being detected in the UK following the tip-off, officials said.
Both devices, which were bound for Chicago, originated in Yemen and are thought to have been made and dispatched by al Qaida.
An anti-terror investigation was focusing on Saudi-born bomb-maker Ibrahim Hassan al-Asiri, believed to be a member of al-Qaida in the Arabian Peninsula (Aqap), as the prime suspect for making the bombs.
He is also believed to have been responsible for making the device involved in the failed Christmas Day bomb plot over Detroit last year.
As part of new security measures brought in following the foiled attacks, countries sending airfreight to the UK will be "graded" according to risk, Transport Secretary Philip Hammond said.
Rules which allowed some freight not to be rescreened if it was merely passing through the UK were being amended, he added.
Ink cartridges larger than 500g (17.6oz) will be banned from hand baggage on flights departing from the UK and also on cargo flights unless they originate from a regular shipper with security arrangements approved by the Department for Transport.
The FBI and Homeland Security Department in the US have warned local officials across the country that packages from abroad with no return address and excessive postage required a second examination.
They added that Aqap was not behind the September 3 crash of a UPS cargo plane in Dubai, despite claims to the contrary from the group itself.
Crash investigators have found no evidence of an explosion on board the UPS cargo plane.