Police have arrested a British businessman on suspicion of terrorism after traces of uranium were found in a package at London's Heathrow Airport.
The radioactive material was found by Border Force officers in a shipment of scrap metal on December 29.
Specialist scanners detected the uranium as it was transported to a freight shed.
On Saturday, specialist officers arrested a man in his 60s in Cheshire, in the north of England, under section nine of the Terrorism Act. He has been bailed until April.
The man, a British citizen, has been questioned over "making or possession of radioactive device or possessing radioactive material with the intention of using it” in the commission or preparation of an act of terrorism.
Searches of a an address in Cheshire have not found any other dangerous materials, police said.
Commander Richard Smith, who leads the Met’s Counter Terrorism Command, said initial investigations have not uncovered an immediate threat to the public.
“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," he said.
“However, detectives are continuing with their inquiries to ensure this is definitely the case.”
A representative from the Met police said the uranium was found during “routine screening” at Heathrow.
“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," Mr Smith said.
“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.”
Uranium can be used for civilian power generation and scientific purposes and is a key ingredient in nuclear weapons.
Certain isotopes emit radiation that can be harmful to humans, and the metal itself is toxic if ingested or inhaled.
Col Hamish de Bretton-Gordon, a chemical and biological weapons expert and former head of the UK’s nuclear defence regiment, said it was reassuring for the public that the security checks in place had worked in detecting it.
“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," he told BBC Radio 4.
“In this country, I think people should be pretty reassured that we’re not going to see dirty bombs from this type of material.
“If it is for nefarious reasons, for bad reasons, to create mayhem by Iranians or some sort of Russian proxy, then that is an area of concern.
“But I think the key thing is that there are people looking out for this and this should not worry the public unduly.”