Police are investigating after metal contaminated with uranium was found at London's Heathrow Airport last month.
Officers of the Metropolitan Police's Counter Terrorism Command responded to the security alert which was triggered on 29 December.
The Sun, which first reported the news, said the uranium came from Pakistan.
One line of inquiry is whether it was the result of "poor handling" in the country, the BBC was told. Police say there was no threat to the public.
It was found in a shipment of scrap metal, a source said.
A Pakistan foreign ministry spokesperson told BBC News that the reports were "not factual", adding that no information to this effect had been shared with Pakistan officially.
A former commander of the UK's defence forces said "a very small sample" was found and offered assurances that "there are people looking out for this 24 hours a day".
Colonel Hamish De Bretton-Gordon said the incident "should not worry the public".
However, he told BBC Radio 4's Today programme that, in light of recent nuclear threats, he could see why the public was concerned.
He said uranium could potentially be used for nuclear fuel in power stations and, when highly enriched, it could be used for nuclear weapons.
Alarms were triggered at Heathrow after specialist scanners detected the substance as it was ferried to a freight shed owned by handling firm Swissport, the Sun said.
The shipment's intended destination is not clear. No-one has been arrested.
The Metropolitan Police said: "We can confirm officers from the Met's Counter Terrorism Command were contacted by Border Force colleagues at Heathrow after a very small amount of contaminated material was identified after routine screening within a package incoming to the UK."
Commander Richard Smith from the force's counter terrorism team separately told the BBC: "Although our investigation remains ongoing, from our inquiries so far, it does not appear to be linked to any direct threat.
"As the public would expect, however, we will continue to follow up on all available lines of inquiry to ensure this is definitely the case."
Strict protocols must be followed in order to fly dangerous cargo, including uranium, being loaded onto the base of units in the cargo hold and ensuring a minimum distance is kept between the nuclear material and cabin above.
Uranium is an element which occurs naturally. It can have nuclear-related uses once it has been refined, or enriched. This is achieved by the use of centrifuges – machines which spin at supersonic speeds.
Low-enriched uranium can be used to produce fuel for commercial nuclear power plants.
Highly enriched uranium has a purity of 20% or more and is used in research reactors. Weapons-grade uranium is 90% enriched or more.
Cabinet minister Steve Barclay said he hoped for more information in "due course" and it was right an investigation "looks at all the issues".
"I'm learning about this this morning," he told Sky News.
The Home Office said: "We do not comment on live investigations."
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