Protesters breached tight security at Britain's parliament on Wednesday, getting on to the roof and unfurling banners protesting against government plans to expand London's Heathrow airport.

One of the banners hanging down the side of the building read "NO THIRD RUNWAY", while another read "BAA HQ", referring to the British Airports Authority which owns Heathrow. BAA is itself owned by Spain's Ferrovial.

The protest by anti-aviation group Plane Stupid coincided with the end of a public consultation on proposals to build a third runway at the world's busiest international airport. Security at parliament was beefed up following the Sept. 11, 2001 attacks in the United States and tightened further after protesters hurled coloured 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.

"This major breach of security is overshadowed by (Prime Minister) Gordon Brown's breach of climate security with these expansion plans," Plane Stupid spokesman Malcolm Carroll said. "It shows the seriousness with which we treat these plans and the kind of direct action we are prepared to take," he said, threatening further disruption.

Carroll said five protesters, who were talking to police on the roof, had entered the main Westminster Hall on a guided tour, taken a lift and reached the roof via a fire escape. The protest follows a stunt on Monday by four Greenpeace activists. They breached security at Heathrow, climbed on to the roof of a parked airliner and tied a banner protesting at the expansion plans on to the aircraft's tail.

Building a third runway at Heathrow has sparked a virulent blogging campaign stressing the contradiction between major aviation expansion and attempts to combat global warming. Heathrow handles 67.3 million passengers and 471,000 aircraft movements a year, figures which are forecast to double over the next 30 years.

Scientists say global average temperatures will rise by between 1.8 and 4.0 degrees Celsius this century due to carbon emissions from burning fossil fuels for transport and power. They also note that emissions at altitude are twice as harmful as those at ground level.

The government argues that aviation expansion is vital for the economy and to meet booming business and private demand. Campaigners say relatively little commerce moves by air and only about one quarter of flights are for business reasons.