A man in his 60s has been arrested under terrorism laws after traces of uranium were found in a cargo package at Heathrow Airport.
Police said the man was arrested in Cheshire on Saturday and later released on bail.
Border Force officers found a small amount of uranium with a shipment of scrap metal following a routine screening at Heathrow Airport on 29 December.
Commander Richard Smith, who leads the Metropolitan Police's counter-terrorism command, said: "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 has been released on bail until a date in April, the force said in a statement.
Officers said they did not find any other dangerous material at the address in Cheshire but the offence the man has been accused of relates to the making or possession of radioactive material.
The uranium had arrived on a flight from Oman after originating in Pakistan, Sky sources have said.
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.