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Heathrow Airport was today investigating a significant security lapse after four Greenpeace protesters succeeded in climbing on the roof of an aircraft which had just landed.

Greenpeace says that the protesters, two male and two female, climbed on top of the fuselage of the British Airways Boeing 777 flight at 9.45am.

The four claimed that they had waited until all the passengers had disembarked from the flight - which had recently arrived after a domestic journey from Manchester - and then walked through double doors at the terminal, crossed an area of runway and climbed the stairs.

Police eventually forced them down from the plane at 11am, and officers said that they were being held at Heathrow's police station. Greenpeace described the demonstration as an "incredible security breach".

In what they said was a protest at a third runway to be planned for Heathrow, the demonstrators unfurled a huge banner on the aircraft tail, saying: "Climate Emergency - No Third Runway."

The incident comes as a huge embarrassment to Heathrow, Europe's busiest airport, which is meant to be among the most secure in the world.

BAA, which runs Heathrow, today described the protest as "unlawful and irresponsible" and added that it was carrying out a "full investigation".

One of the protesters, Sarah Shoraka, 30, from London, said that she was surprised how easily she and her three colleagues climbed on to the plane.

"We have exposed a hole in security but what we are trying to do is expose a hole in the argument for expanding Heathrow," she said.

"We are not going to be able to meet our climate change obligations if we expand airports."

Another of the protesters, Anna Jones, 27, from Leeds, said: "I am standing on this plane because our planet and the people who live on it are in danger."

The protest came two days before the end of a Government consultation into Heathrow expansion. Greenpeace claimed the process of consultation was "fixed".

A BAA spokesman said that airport operations were not affected by the breach.

"There is an important debate to be had regarding airport expansion, and BAA respects people's democratic right to protest lawfully. However direct action on the airfield is unlawful and irresponsible.

"The Government is currently consulting on the future of Heathrow Airport and all parties have the opportunity, through the proper democratic process, to make their views known.

"We are working with the police, the airline and the relevant authorities to carry out a full investigation."

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