A major security alert has taken place at Heathrow airport after a man with a rucksack scaled the perimeter fence and ran into the path of an aircraft. Eyewitnesses told the BBC armed police surrounded the man on the north runway. Scotland Yard said a man was arrested and a controlled explosion of the bag found nothing suspicious. They later confirmed it was not terrorism-related.
Gordon Brown said everything was being done to ensure security at the airport, as some passengers experienced delays.
The BBC's Ben Ando said police received numerous 999 calls at 1405 GMT from members of the public. Reports were of a man of Asian or Arab appearance removing his shoes before scaling the airport's perimeter fence, he said.
Up to 20 emergency services vehicles were reported to have been sent to the scene. The rucksack was recovered and explosives officers called in as a precaution. A Metropolitan Police spokeswoman said: "A man was arrested and is currently in custody."
The security breach comes a day before the Queen is due to open the new £4.3bn Terminal 5 building.
Norman Shanks, a former head of security at airports operator BAA, said it was not the first time a person had climbed over airport fencing. He said it often transpired they were drunk or mentally unstable, and occasionally they were protesters. "Who knows what category this person falls into?" he told BBC News 24.
He also said that - while it was possible to climb such barriers - airports also have patrols, surveillance and fast-response units in place.
Last month protesters opposing airport expansion plans breached security to unveil a banner on top of a parked passenger jet. Campaign groups Greenpeace and Plane Stupid both denied any involvement in this latest incident.
Aviation security expert Chris Yates described it as "extraordinarily serious" and said a lot of hard questions would now be asked of Heathrow's security.
The northern runway, also identified as 27R, was temporarily closed but is now fully operational. The southern runway remained open throughout the incident.