A small amount of uranium was detected in a package that arrived in the UK at Heathrow Airport following a routine screening.
The Metropolitan Police said its counter-terrorism command unit was contacted by Border Force colleagues at the airport after the contaminated material was discovered on 29 December.
Counter-terror commander Richard Smith said the amount of contaminated material "was extremely small" and has been assessed by experts as posing no threat to the public.
The package was destined for Iranian nationals in the UK and arrived on a flight from Oman after originating in Pakistan, according to The Sun.
It was found in a shipment of scrap metal, the BBC reported.
Mr Smith added: "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 enquiry to ensure this is definitely the case.
"However, it does highlight the excellent capability we and our partners have in place to monitor our ports and borders in order to keep the public safe from any potential threats to their safety and security that might be coming into the UK."
The material has been identified as being contaminated with uranium, the force said, and no arrests have been made.
Former head of the British Army's chemical weapons unit, Hamish de Bretton-Gordon, told Sky News that it was "a concern" that the material has apparently reached the UK, having come all the way from Pakistan, but that he did not think the public should be overly worried.
He added: "I think it's excellent that the police and others have interdicted this and made it safe, but we must be on our guard because there are bad people out there who want to do us harm in this particular manner."
Speculating about the intent, he said "we must be open to the fact this might be some sort of terror type thing".
Mr Bretton-Gordon said while there is no indication that a group such as Al Qaeda were behind the incident, he argued it "has their trademark and fingerprints on it".
Whatever the origin, he said the material "absolutely shouldn't be on a commercial airliner".
The Met said officers are working with partner agencies to investigate the incident and ensure there is no risk to the public.
A Home Office spokesperson said: "We do not comment on live investigations."
Uranium is a metal that exists naturally in the earth, but is harmful to humans because it is radioactive.
Heath Secretary Steve Barclay told Sky News that he hoped for more information in "due course" about the material seized at Heathrow.
He said: "Clearly there is an investigation under way and it is right that it looks at all the issues, and I'm sure it will report in due course."