Protesters breached security at Westminster today by climbing on to the roof of the Houses of Parliament to demonstrate against the expansion of Heathrow airport.
The five activists from campaign group Plane Stupid unfurled two huge banners, one saying No Third Runway, and the other, in a reference to airport operator BAA, read BAA Headquarters.
The protest came on the final day of the Government's consultation on expanding Heathrow airport, including a third runway.
Plane Stupid said they gained access to Parliament as visitors, walked through the building, got into a lift and then climbed onto the roof.
It was the second security breach this week following an incident at Heathrow on Monday when five Greenpeace activists managed to climb on top of a British Airways plane in protest against expanding the airport.
The protest lasted almost three hours and ended shortly after Gordon Brown stood up in the Commons at the start of Prime Minister's Questions.
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.
"It is a very important message that should be sent out to those people who are protesting."
One of the roof protesters, Richard George, 27, from London, said: "I am stood on the roof of Parliament because the democratic process had been corrupted.
"The aviation industry had 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.
"The Prime Minister does not even have the courage to ask Londoners the simple question: 'Do you want a third runway?'. Instead, his Government published a consultation document full of gobbledegook and industry spin."
Mr George added: "We decided to let Gordon Brown get on with Prime Minister's Questions, but we just wanted him to know what it is like to have an inconvenience above your head that you did not ask for."
They made paper aeroplanes out of confidential Whitehall documents they claimed showed the consultation process was "fixed", and glided the planes into the MPs' car park below.
The campaign 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 at the world's biggest international airport.
Mr George added: "Now the consultation is over, we can safely ignore the fixed result and get on with the job of stopping this new runway being built.
"A huge coalition of local residents, Londoners and environmentalists is coming together, supported by all the major mayoral candidates, to stand against Gordon Brown and say 'no more'."
Plane Stupid said BAA and the Government wanted a sixth terminal and third runway built over homes, schools and churches in the villages of Sipson and Harmondsworth.
"This would increase the number of flights from 480,000 a year to at least 702,000.
"Two million Londoners face increased levels of noise, while CO2 emissions from the airport would shoot up despite claims by Brown that he's committed to fighting climate change," a spokesman said.
The protesters said they branded Parliament "BAA HQ" because of the "extraordinary level of collusion" between the aviation industry and government.
Matthew Knowles, spokesman for the Society of British Aerospace Companies, said: "These stunts are becoming tiresome and do nothing more than peddle inaccurate propaganda.
"The aviation industry has achieved a 75% cut in fuel burn over the past 50 years and a similar reduction in noise from aircraft in the past 30 years."
Michelle Di Leo, director of FlyingMatters, a coalition of unions, business, tourism groups and farmers, commented: "Today's action by Plane Stupid is misjudged, misdirected, and irresponsible.
"Aviation is responsible for 2% of global carbon emissions and is growing at a slower global rate than power generation and industry.
"If they are serious about climate change, they should engage in a proper debate about solutions that will make a real difference rather than indulge in publicity stunts which waste police time."
Westminster sources said they suspected the banners were stored inside the Commons for the protesters to collect this morning.
The banners were believed to be too big for the activists to have taken them through security, it was suggested.