Aviation experts believe the incident at Heathrow is bound to lead to even tighter security at the west London airport.

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

He went on: "This is the sort of thing that can happen at airports overseas, particularly those where security is not tight."

A man has been arrested after he was seen running onto the northern runway.

The intruder scaled a perimeter fence carrying a bag, according to Heathrow workforce sources. He ran towards a plane, dumped the bag and was quickly tackled by armed police.

There was a controlled explosion on the bags and sources said early reports indicated that no explosives were found.

One theory amongst Heathrow workers was that the man was a protester bent on causing disruption ahead of Friday's opening by the Queen of the airport's new Terminal 5.

Scotland Yard said anti-terrorist officers were aware of the incident but it was being dealt with by local police at the airport.

It is the second time in 18 days 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.

But Greenpeace and climate change protest group Plane Stupid denied any involvement.

A spokesman for British Airways said flights were being disrupted because the runway involved in the incident has been closed.

A number of police vehicles and a fire engine could be seen on the runway.

British Airways said it would have to make a "small number" of cancellations because of the security breach but not a "significant number". No specific figures were immediately available.

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