The UPS flight travelled to Istanbul with the package, reportedly containing a timer, wires and a detonator.
The UK Department for Transport said it was taking the matter "very seriously".
A 26-year-old man was arrested at his home last week on suspicion of making a bomb hoax but later bailed to return to a north London police station in May 2011.
Police stressed the incident was not terrorist-related.
A Turkish man had delivered the package, disguised as a wedding cake box, to a UPS office in Camden, north London, two weeks ago, according to ITV News.
A Department for Transport spokesman said: "We have already begun an investigation which will look at all aspects of this incident, including UPS's procedures.
"The UK has one of the toughest security regimes for air cargo in the world. All security measures are subject to continuous review."
"UPS is co-operating with the UK Department for Transport's investigation of the incident. UPS has a multiple-layered approach to ensure security."
Last October a bomb disguised as an ink cartridge was found on a UPS cargo plane at East Midlands Airport.
It had been timed to detonate over the eastern seaboard of the US, police later said. The bomb, and another found in Dubai, contained at least 300g of the explosive PETN.