A small amount of uranium was detected in a package that arrived in the UK at Heathrow airport after routine screening.
The Metropolitan Police said its counter-terrorism commando unit was contacted by Border Force colleagues at the airport after the contaminated material was discovered on December 29.
Counter-terrorism commander Richard Smith said the amount of contaminated material “was extremely small” and experts have assessed that it does not pose a threat to the public.
The package was destined for Iranian citizens in the UK and arrived on a flight from Oman after originating in Pakistan, according to The Sun.
It was found in a junk shipment, the BBC reported.
Smith added: “Although our investigation is ongoing, based on our investigations thus far, it does not appear to be linked to any direct threat.
“However, as the public would expect, we will continue to pursue all available lines of inquiry to ensure that this is definitely the case.
“However, it highlights the excellent ability we and our partners have to monitor our ports and borders in order to keep the public safe from any potential threat to their security that may enter the UK.”
The material has been identified as being contaminated with uranium, the force said, and no arrests have been made.
The former head of the British Army’s chemical weapons unit, Hamish de Bretton-Gordon, told Sky News it was “a concern” that the material had apparently reached the UK, having come from Pakistan, but that he did not believe the the public should be too concerned.
He added: “I think it’s great that the police and others have intercepted this and made it secure, but we have to be on our guard because there are bad people out there who want to hurt us in this particular way.”
Speculating on the intent, he said that “we have to be open to the fact that this could be some kind of horror.”
Bretton-Gordon said that while there is no indication that a group like al Qaeda was behind the incident, he argued that “it has their trademark and fingerprints on it.”
Whatever the origin, he said the material “absolutely should not be on a commercial aircraft.”
The Met said officials are working with partner agencies to investigate the incident and ensure there is no risk to the public.
A Home Office spokesman said: “We do not comment on live investigations.”
Uranium is a metal that exists naturally on earth, but it is harmful to humans because it is radioactive.
Health Secretary Steve Barclay told Sky News he hoped to get more information “in due course” about the material seized at Heathrow.
He said: “There is clearly an investigation going on and it is right for me to look at all the issues, and I am sure it will be reported in due course.”