A man was arrested by armed police after sprinting into the path of a plane on a runway at Heathrow Airport today, triggering a major security alert on the eve of a visit by the Queen to Britain's busiest airport.

Witnesses said that the man, carrying a rucksack, clambered over a fence onto Heathrow's northern runway just after 2pm. He dumped the bags on the grass perimeter strip before running into the path of an Emirates Boeing 777 and was quickly surrounded by armed police.

The Metropolitan Police said that a man was being held in custody. The bag was destroyed in a controlled explosion but is not thought to have contained any explosive device.

One theory is that the man was a protester hoping to derail tomorrow's ceremony to open Heathrow's new Terminal 5 building, which the Queen is due to attend. A spokesman for the climate change protest group Plane Stupid denied any involvement.

Today’s incident is the second time in recent weeks that security has been breached at the airport. Greenpeace protesters walked on to the runway late last month and climbed on to a Boeing 777, before unfurling a banner in protest against the airport’s expansion. Two days later, a group of Plane Stupid protesters bypassed security at Parliament and climbed onto the roof of the Palace of Westminster.

BAA, the airport operator, said that the northern runway - usually used for take-offs - remained partially closed after today's arrest. The southern runway is open as usual and is being used for both take-offs and landings.

British Airways said it would have to make a "small number" of cancellations because of the security breach, but not a "significant number".

Witnesses to this afternoon's incident spoke of a rapid response by the emergency services. A number of police vehicles and a fire engine sped onto the runway after the intruder was spotted. Officers seized the man and held him down on the grass at gunpoint before leading him away.

"I’m satisfied everything is now being done to ensure security at Heathrow is intact. We are determined to protect all passengers and all staff who go through Heathrow and every other airport."

Aviation experts predicted that the scare was bound to lead to even tighter security at the west London airport.

According to Max Kingsley-Jones, deputy editor of Flight International magazine, the incursion on to the runway might even result in the military being called in to patrol the airport’s perimeters.

He said: "This is the sort of thing that can happen at airports overseas, particularly those where security is not tight. However, for it to happen at the world’s biggest international airport is not good."

Mr Kingsley-Jones went on: "Heathrow has a road all the way around it and a 15ft-20ft high fence that has probably not changed for about 30 years. Heathrow police make regular patrols around the perimeter fence, and also police and staff from airport operator BAA do patrols inside, although BAA people are mainly checking for obstacles that could be hazardous for aircraft.

"Certainly, people who run the airport and people who use it will reckon this incident is not good. In 1994, the IRA fired a mortar bomb at the northern runway and this led to troops patrolling at Heathrow."

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