Security at one of Britain's busiest airports is under urgent review tonight after environmental campaigners broke in, forcing the runway to be closed.
More than 50 flights were cancelled and many others were delayed after dozens of activists from protest group Plane Stupid cut through Stansted Airport's security fence at around 3am.
Police arrested 57 people and the runway reopened just after 8am but disruption to flights continued throughout the day.
Ryanair, the only airline to cancel flights, called for an investigation into why airport operator BAA had failed to keep the Essex airport "secure and open", saying the disruption to passengers was "unacceptable".
Spokesman Stephen McNamara said: "The security at BAA Stansted airport has once again failed.
"We need to know what measures the BAA is putting in place to prevent these repeated security failures."
Long queues formed at Ryanair check-in desks today and armed police officers helped to keep order as a small number of passengers became irate.
Some of those queuing said they were alarmed protestors had got in so easily.
Terry Purton, 61, of Edenbridge, Kent, said: "If they can do it, I'm sure there would be nothing to stop terrorists."
Lainey Mace, of Fakenham, Norfolk, added: "One would have thought security at an airport would be much tighter than that."
Stansted Airport and Essex Police said they will investigate the security breach.
"If there are lessons to be learnt, that is what we need to take on board. We need to discover if there's anything we can do better," a BAA spokesman said.
"This is a site of about 2,000 acres bordered by a security fence. It is not operated as a fortress."
He said the protestors were quickly intercepted and a second group was prevented from going through the fence.
Nick Barton, the airport's commercial and development director, added: "At no time were any passengers or planes in danger."
Today's protest was sparked by the Government's decision to allow the expansion of the airport with a second runway.
Activist Lily Kember, 21, a third-year anthropology student at Edinburgh University, said the group used bolt-cutters to get into a secure area around 50 metres from the runway.
Ms Kember, from London, said the threat of climate change was more "terrifying" than the fear of arrest.
A spokesman for Plane Stupid said tonight: "We have to take these necessary actions. It was a big action, it's a big problem and a strong message."
Today's breach follows a similar climate change protest at Heathrow Airport, west London, in February when Greenpeace protestors climbed on top of a British Airways plane and unfurled a banner protesting against a third runway.
Greenpeace today supported Plane Stupid, saying: "The climate change secretary Ed Miliband called for a Suffragette-style movement to pressure governments to act. Well, he got his wish.
"The Suffragettes were disruptive and lambasted by the establishment of the day, but have been utterly vindicated by history, and no doubt it will be the same with Plane Stupid."
Mick Rix of the GMB union, which represents Stansted Airport staff including baggage handlers and check-in workers, said: "The GMB has been raising the state of the fencing around Stansted Airport for a number of months because we have not been satisfied that it is in a good enough state.
"It was obvious to GMB that protesters would seek to breach it.
"We believe the perimeter of the airport should be secured by a quasi-official airport security force rather than by a conglomerate of security contractors. Since it is a criminal offence to breach this fence, those guarding it should have powers of arrest."
Essex Police said 50 of those arrested were held on suspicion of aggravated trespass, three on suspicion of attempting to gain access to a restricted area and four of conspiracy to commit a public nuisance. They remain in custody tonight.
Climate change was cited as one of the reasons for refusal when the local planning authority Uttlesford District Council turned down BAA's application to expand Stansted in November 2006.
Environmental campaigners hailed the decision as a benchmark saying it was the first time climate change had been listed as a reason for refusing a planning application.
The Government overruled Uttlesford District Council in October this year following an appeal by BAA.
Transport Secretary Geoff Hoon granted permission for the airport to increase the number of flights from 241,000 to 264,000 and raise the number of passengers from 25 million to 35 million.