LONDON (Reuters) - Protesters scaled the roof of Britain's parliament in a major security breach on Wednesday and threatened further direct action against government plans to expand London's Heathrow airport.
Environmental protesters from the "Plane Stupid" group scaled the Houses of Parliament to mark the end of a public consultation period on a third runway and sixth terminal at what is already the world's busiest international airport.
"NO THIRD RUNWAY," read one of the banners they unfurled and hung down the side of the building.
The demonstration followed another serious breach on Monday when Greenpeace activists penetrated Heathrow's own security to climb on an aircraft and wrap a banner around its tail plane.
"This is all about no third runway," Plane Stupid spokesman Malcolm Carroll said. "The direct action movement know we have got to take these protests to another level to get the government to listen."
Security at parliament was beefed up after the September 11, 2001 attacks in the United States and tightened further when fathers' rights protesters hurled colored flour at then Prime Minister Tony Blair in May 2004 while he was in the debating chamber.
Britain sought to tighten security again at public buildings after suicide bombers killed 52 people in attacks on London's transport network in July 2005.
The government, business groups, airlines and airport owner British Airports Authority say Heathrow must either expand or risk losing out to continental rivals serving emerging destinations in India and China and damaging the economy.
But environmental groups have joined forces with angry local residents, saying roughly doubling the number of flights from Britain's busiest air hub would make a mockery of government pledges to reduce carbon emissions to tackle global warming.
Heathrow already handles 67.3 million passengers and 471,000 aircraft movements a year.
"This major breach of security is overshadowed by (Prime Minister) Gordon Brown's breach of climate security with these expansion plans," said Carroll.
Brown's spokesman said the decision to build a third runway had been made in principle as long as strict noise and environmental conditions were met. And, speaking in parliament as the protest continued above his head, Brown insisted:
"Decisions in this country should be made in the chamber of this house and not on the roof of this house."
The campaigners, who were escorted down from the roof by police just before 1230 GMT after a protest lasting two hours, say the Heathrow consultation was a sham and pin their hopes on the government being swayed by public pressure.
Residents' groups, also furious over increased noise pollution and the demolition of some 700 homes around the airport, say they are preparing court action and if necessary will try and block the bulldozers if expansion goes ahead.
They are supported by London Mayor Ken Livingstone, a string of local councils and Britain's third-largest political party the Liberal Democrats.
BAA says it hopes it can work with all sides if expansion goes ahead and wants to avoid the kind of arguments that delayed the building of the airport's Terminal Five and left the existing facilities badly overstretched.
Additional reporting by David Cutler. Editing by Kate Kelland