Inside job: Police hunt mole who guided green protesters onto roof of the Commons

Lobbing paper aeroplanes into the sky, five protesters perched on top of the Houses of Parliament yesterday in another embarrassing breach of Westminster security.

They spent almost three hours on the roof demonstrating against Heathrow airport expansion plans before being arrested and led off to be questioned by police.

Gordon Brown criticised the activists, telling MPs: "Decisions in this country should be made in the chamber of this House and not on the roof of this House."

But the ease with which the three men and two women from campaign group Plane Stupid managed to get on to the rooftop with large banners and handcuffs raised new questions about protecting Parliament.

With terrorism fears running high, barricades have been erected around the buildings and extra CCTV cameras installed.

Yet the five – two of whom disrupted a parliamentary committee meeting three months ago – claim they entered separately as visitors and met up inside Parliament before getting on to the roof, via a spot where Westminster workers go for a cigarette.

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Protester Richard George, speaking by mobile phone from the shadow of Big Ben, said: "We went up in a lift and walked out on to the roof. It was as simple as that."

Parliamentary officials and police will investigate whether the five had inside help with their high-profile demonstration – the second group were Mr George, 27, co-founder of Plane Stupid, Cambridge graduate Tamsin Omond, 23, who hopes to be ordained as a priest, Olivia Chessell, 20, a youth arts co-ordinator, Graham Thompson, 34, another co-founder of Plane Stupid, and Leo Murray, 31, an award-winning animator.

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Their protest came on the final day of the Government's consultation on expanding Heathrow airport, including a third runway.

The protesters made paper aeroplanes out of confidential Whitehall documents which they claimed showed the consultation process was "fixed".

They said the papers proved that the Government had already decided to build a third runway and a sixth terminal at the world's biggest international airport.

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Mr George said: "The aviation industry has taken full advantage of a weak Prime Minister to get the Heathrow consultation fixed.

"It does not even consider global warming despite everything Brown has said about the environment and despite the massive impact aviation has on the climate."

But Matthew Knowles, spokesman for the Society of British Aerospace Companies, said: "Heathrow's expansion is of national importance if jobs are to be safeguarded and created, both locally as well as in the City and across the country."