A man has been arrested under terrorism laws after traces of uranium were found in a cargo package at Heathrow Airport, police said.
The radioactive material was found by Border Force officers with a shipment of scrap metal on 29 December.
The man, in his 60s, was detained by police in Cheshire on Saturday under Section nine of the Terrorism Act. He has been bailed until April.
Commander Richard Smith, who leads the Met’s Counter Terrorism Command, said: "The discovery of what was a very small amount of uranium within a package at Heathrow Airport is clearly of concern, but it shows the effectiveness of the procedures and checks in place with our partners to detect this type of material.
"Our priority since launching our investigation has been to ensure that there is no linked direct threat to the public. To this end, we are following every possible line of inquiry available to us, which has led us to making this arrest over the weekend.
Watch: Key questions remain after uranium discovered at Heathrow Airport
"I want to be clear that despite making this arrest, and based on what we currently know, this incident still does not appear to be linked to any direct threat to the public.
"However, detectives are continuing with their inquiries to ensure this is definitely the case."
The man was arrested after counter-terror officers searched an address in Cheshire on Saturday.
He was arrested under Section 9 of the Terrorism Act 2006, which covers the making and possession of radioactive devices.
The package arrived on a flight from Oman after originating in Pakistan, The Sun newspaper reported, and was said to be destined for Iranian nationals in the UK.
Chemical and biological weapons expert Colonel Hamish de Bretton-Gordon, former head of the UK’s nuclear defence regiment, told BBC Radio 4 on Wednesday that people should be reassured that the uranium was detected.
He said: "It’s very clear that the comprehensive surveillance network that we have in place in this country, run by the security services, the police and others, has actually worked and picked up potentially a very dangerous containment that could provide a threat.
"In this country I think people should be pretty reassured that we’re not going to see dirty bombs from this type of material."