The four campaigners boarded the 8.15am flight from Manchester to Heathrow carrying high visibility vests and a banner stating "climate emergency - no third runway".
On arrival at Heathrow, the Greenpeace members were the last passengers to disembark from the plane at gate five, at the far end of Heathrow's Terminal 1.
Once off the aircraft, they ducked through a door next to a branch of Costa Coffee and descended stairs to a set of double doors on the ground floor next to the BA A320 plane.
The protesters pushed through the door, which was held in place by a magnet lock, before clambering up mobile stairs attached to the air bridge - used by passengers to get on and off the plane - and climbing on to the top of the jet, where they hoisted their banner.
It is understood that Greenpeace identified the route to the airport apron on a recent reconnaissance trip. One of the protesters remained on the ground to assure BA and BAA staff that the stunt was peaceful.
A Greenpeace spokesman said the protest had highlighted a "shocking hole" in BAA security at Heathrow, but added that the demonstration was intended to highlight the greater hole in the government's aviation policy, which sanctions a third runway at Heathrow.
A spokesman for BAA, the airport's operator, said: "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."
The GMB union called for an investigation into whether the increasing use of agency staff by BAA had contributed to the security breach.
A spokesman for Manchester airport said the protesters were not stopped from boarding the plane because they were not carrying illegal items.
He could not confirm whether security staff at Manchester had discovered the banner but allowed the Greenpeace members to board the aircraft.
"They were fully ticketed passengers, so if they had the banner in a bag or suitcase and it did not contain anything illegal, it would be put through as normal baggage," he said.
© 2008 The Guardian