Heathrow expansion protesters who sparked a major security breach after climbing on to the roof of the Houses of Parliament this morning may have been helped by someone on the inside, according to police.

The six activists from campaign group Plane Stupid began their protest at around 9.45am and after unfurling two huge banners, handcuffed themselves to the railings.

A witness said that the protesters must have gained access to the building with someone who had a security card and speculated that they must have had help in finding their way up onto the roof.

Scotland Yard said in a statement: "Police were called at approximately 9.30am to reports of a group of protesters on the roof of the Houses of Parliament.

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"It is believed there are six protesters who are currently in dialogue with officers at the scene.

"At this early stage we believe there is a strong possibility the protesters are guests of pass-holders."

As boltcutters attempted to free the protestors, they waved at cameras and threw paper aeroplanes from the roof.

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

One of the banners said No Third Runway, and the other, in a reference to airport operator BAA, which read BAA Headquarters.

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