LONDON, England (CNN) -- Four Greenpeace activists breached security at Heathrow Airport on Monday to climb on top of a British Airways plane and protest plans to build a third runway, the environmental group and airport authorities said.

The four activists were atop the Airbus A320 for about an hour and a half before airport security plucked them off in a cherry-picker crane and police arrested them, authorities said.

Greenpeace said it wanted to highlight the environmental damage a third runway will cause, but a Heathrow spokesman called the group's actions "irresponsible."

"Climate change can be beaten, but not by almost doubling the size of the world's biggest airport," said protester Anna Jones, 27.

She and the other three activists -- a woman and two men -- unfurled a yellow banner on the plane's tailfin which read, "Climate Emergency -- No 3rd Runway." Greenpeace said it had planned the protest for many months.

Greenpeace said the activists bought airline tickets to get airside at Heathrow's Terminal 1, then waited for all the passengers on the Manchester, England to London flight to disembark. The activists then walked through the terminal, crossed the Tarmac, and climbed stairs onto the plane's fuselage.

"We may have exposed a hole in security at Heathrow, but it's not as big as the hole in (Prime Minister) Gordon Brown's climate change policy," said Sarah Shoraka, 30, another of the protesters.

Shoraka said the group chose the Manchester-to-London flight because the train is a viable and more environmentally-friendly alternative on the same route.

"The push for a third runway is being fueled by totally unnecessary flights like this one," Shoraka said.

A Heathrow spokesman who spoke on condition of anonymity said authorities were investigating the "unlawful protest."

Wednesday is the deadline for the government's public consultation on whether a third runway should be built.

"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," the spokesman said.

Heathrow is the main airport in Britain and one of the biggest in the world. But despite its size Heathrow has only two runways, and the government and airlines have for years called for the construction of a third.

Opponents complain of the environmental impact, noise, pollution, and destruction to local property a third runway would create.

Shoraka said the protest was the only way to get the group's message across.

"The consultation that the government has set up has been widely derided as a fix," she said. "We're reasonable people but we just felt we had to take this action today because there's nothing else we can do within our power to stop climate change." E-mail to a friend

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