A Sudanese man stopped at Heathrow Airport for allegedly carrying five bullets in his coat has been held under the Terrorism Act.
No flights were disrupted by the security scare
The 45-year-old man, who flew into Heathrow from Washington DC in transit to Dubai, was initially arrested under the Firearms Act.
The man was picked up by routine airport checks on Wednesday morning.
He was taken to a central London station to be interviewed by anti-terror police on Wednesday night.
The British airline pilots' union, Balpa, says "questions will be asked" about security at the Washington end.
The man was questioned earlier by Heathrow police, while forensic experts examined the ammunition that security staff said they found on him.
Passengers who came in on the same flight said the man told staff he did not realise the items were in his coat pocket.
He arrived at the airport's Terminal 3 at 0740 GMT on a Virgin Atlantic flight, police said.
Earlier, a police source said the man was not a known terrorist suspect.
A Virgin Atlantic spokeswoman confirmed the man had flown from Washington
Dulles to Heathrow on flight VS022 in transit to
board a flight to Dubai with another airline .
The airline said in a statement the "item" seized did not pose a threat to the aircraft.
The statement read: "The safety and welfare of our passengers and crew is Virgin Atlantic's top priority.
"Screening of passengers at Washington Dulles airport is the responsibility
of the Transportation Security Administration."
A TSA spokesman said "pieces of ammunition" had been found on a man at Heathrow and that the TSA would be investigating the incident.
The United States Department of Homeland Security was not
available for comment.
Wednesday's incident follows American insistence that international flights to or over the US would be required to carry armed guards in certain cases over the festive period.
The decision came amid renewed fears that terrorists may be trying to use aircraft to stage a new attack.
The US request was followed by repeated disruption of the British Airways BA223 flight from Heathrow to Washington.
The flight was cancelled on 1 and 2 January and delayed on each of the next five days amid security fears and under guidance from the government.
A spokesman for British airline pilots' union Balpa said the detection of the man showed that security on the ground was crucial.
He said: "This is where we need to concentrate our investment, and not be diverted
into cul-de-sacs like sky marshals.
"Obviously there will be a lot of questions asked in America as to how he got
on the plane undetected."
David Learmount, operations and security director at Flight International
magazine, said security was ultimately dependent on human vigilance.
"You only need vigilance to lapse for one piece of luggage and that bag gets
through", he said.