A number of Greenpeace activists today climbed on top of a Boeing aeroplane at Heathrow Airport as part of a climate change protest, the environment group said.
Greenpeace described it as an "incredible security breach" and said the activists would be unfurling banners from on top of the Boeing 777 aircraft.
A spokesman said the incident took place at Terminal 1 shortly after the plane had landed from Manchester.
Greenpeace said the plane involved was a British Airways flight from Manchester to Heathrow.
A spokesman added: "The whole area is now surrounded by police."
Greenpeace said the incident took place at 9.45am, adding that the protesters were covering the plane's tailfin with a huge banner that read: "Climate Emergency - No Third Runway."
The two women and two male activists waited until all the passengers had disembarked from the one-hour flight before walking through double doors at the terminal, crossing an area of tarmac and climbing stairs on to the fuselage.
One of the protesters, Anna Jones, 27, from Leeds, said: "I am standing on this plane because our planet and the people who live on it are in danger.
"Climate change can be beaten but not by almost doubling the size of the world's biggest airport.
"The scientists say we only have 100 months to get emissions down so we are here to draw a line in the sand and tell Gordon Brown his new runway must not and will not be built.
"I have never done a protest like this before but people need to take a stand and tell the Prime Minister he cannot ignore the science any longer."
The protest came two days before the end of a Government consultation into Heathrow expansion. Greenpeace claimed the process of consultation was "fixed".
One of the protesters, Sarah Shoraka, 30, from London, said she was surprised how easily she and her three colleagues climbed on to the plane.
"We have exposed a hole in security but what we are trying to do is expose a hole in the argument for expanding Heathrow.
"We are not going to be able to meet our climate change obligations if we expand airports.
"Aviation is a huge polluter and yet the Government wants to double the size of Heathrow which is already massive. It is completely crazy."
She said the protesters were wrapping a banner around the tail fin of the plane and would soon be ending their protest.
A spokeswoman for BAA Heathrow said: "We can confirm that the police are attending an incident involving four individuals carrying out an unlawful protest at Heathrow Airport.
"There is an important debate to be had regarding airport expansion and BAA respects people's democratic right to protest lawfully. However, direct action on the airfield is unlawful and irresponsible.
"The Government is currently consulting on the future of Heathrow airport and all parties have the opportunity, through the proper democratic process, to make their views known.
"We are working with the police, the airline and the relevant authorities to carry out a full investigation. Airport operations are unaffected."