MPs and police were last night asking whether insiders had helped protesters to breach security by climbing on to the roof of the Commons.

Five activists embarrassed parliamentary authorities by appearing on the roof near Big Ben to unfurl banners attacking the planned expansion of Heathrow. The protest came on the last day of the Government’s consultation.

The three men and two women also handcuffed themselves to the building. Police sources said that the banners and handcuffs would have shown up on scanning devices through which all nonpassholders have to go. A passholder may have brought in the equipment and helped the protesters to find their way through the labyrinth of corridors to the roof.

Plane Stupid said that its protesters gained access to Parliament as visitors, walked through the building, got into a lift and then climbed on the roof.

It was the second security breach this week. Five Greenpeace activists climbed on top of a BA plane at Heathrow on Monday.

Yesterday’s incident was the worst breach of Commons security since Otis Ferry led a group of pro-hunt protesters into the House in 2004.

The protest lasted almost three hours and ended shortly after Gordon Brown stood in the Commons for Prime Minister’s Questions. The five were arrested peacefully.

Mr Brown told MPs: “The message should go out today very clearly that decisions in this country should be made in the chamber of this House and not on the roof of this House.”

MPs called for an immediate security inquiry. The Conservative Michael Jack said that the protesters may have had some inside help. “It’s not the easiest of places to find your way around, and for getting on to a roof, I can only think of one particular route, so it may be a bit of an inside track on this one.”

One protester, Richard George, 27, from London, said: “I am stood on the roof of Parliament because the democratic process has been corrupted.

“The aviation industry has taken full advantage of a weak prime minister to get the Heathrow consultation fixed.”

The activists made paper aeroplanes out of confidential Whitehall documents that they claimed showed the consultation process was rigged.The group claimed that the documents, obtained under the Freedom of Information Act, proved that BAA wrote parts of the consultation document and that the Government had already decided to build a third runway and a sixth terminal.

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