A Venezuelan national has been arrested at Gatwick Airport after a live grenade was found in his luggage.
The 37-year-old man had arrived on a British Airways flight from the Columbian capital, Bogota.
He was picked up by Sussex police as he entered Gatwick's North Terminal and held under terrorism laws.
The Terminal was immediately closed and flights suspended.
Earlier, two men were arrested near Heathrow Airport's perimeter fence and held as a "precautionary measure."
Home Secretary David Blunkett said the two incidents proved the threat from terrorists was genuine and had not been made up.
No weapons were found on the Heathrow pair but officers were not satisfied with the answers they gave. They were believed to be of North African origin.
They were taken to Paddington Green police station in central London where they are being held under the Terrorism Act 2000.
Sources said they were not thought to be linked to any plot to attack Heathrow.
The airport has been centre of a huge security operation amid fears terrorists were planning to use rocket-launcher to blow up a plane.
Around 450 heavily-armed troops and hundreds of police officers have been guarding it since Tuesday after what the Government called a "specific threat".
Police have been stopping and searching cars in surrounding areas.
Army Lynx helicopters have been patrolling the airspace around Heathrow while an RAF Nimrod plane has been circling the capital.
Meanwhile, it has been announced the main access road to Stansted Airport in Essex is being closed tomorrow as a security measure.
Army Lynx helicopters have been patrolling the airspace around Heathrow while an RAF Nimrod plane has been circling the capital.