Heathrow counter-terrorism police are investigating a deadly shipment of uranium smuggled into the UK.
Scotland Yard has confirmed Met Counter Terrorism Command officers were contacted by Border Force at the London airport after a material was found to be contaminated during a "routine screening".
The incident happened on December 29 last year.
The constabulary has reassured that the "extremely small" amount discovered poses "no threat".
However, an investigation remains ongoing, with various lines of inquiry being followed, it added.
No arrests have been made and officers are working with partner agencies to investigate and ensure there is no wider risk to the public.
The Sun reports the package was destined for Iranian nationals in the UK, originated from Pakistan and arrived on a flight from Oman.
It arrived at Terminal 4 on an Oman Air flight from Muscat.
A source told the newspaper: “The race is on to trace everyone involved.”
The Mail reports that the material was not weapons grade.
A source said: "The package contained kilos of uranium - but it was not weapons-grade.."
Scotland Yard 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 on December 29, 2022."
Commander Richard Smith said: "I want to reassure the public that the amount of contaminated material was extremely small and has been assessed by experts as posing no threat to the public.
"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."
The force said the material has been identified as being contaminated with uranium.
Mr Smith added that the response highlighted the "excellent capability" the force and its partners had to monitor the UK's ports and borders.
A Home Office spokesperson said: “We do not comment on live investigations.”
There is understood to be no threat to the public according to official assessments.
