Environmental activists briefly stopped flights at London's Stansted airport yesterday after breaking through to the runway, raising security concerns at Britain's third-busiest airport.

More than 50 flights, all operated by budget airline Ryanair, were cancelled before the runway reopened at about 8.15 a.m. (0810 GMT), said a spokeswoman for the airport.

Police said they had arrested 57 protesters, many of whom had chained themselves to fencing they had carried on to the runway and then used to build a temporary stockade.

Ryanair called the disruption an "unacceptable breach of security and management failure" at the airport and called for an inquiry.

Climate change group Plane Stupid, who organised the demonstration, are opposed to plans by operator BAA to expand Stansted.

BAA, a subsidiary of Spanish infrastructure group Ferrovial, has requested planning permission to add a second runway and in October won government permission to increase the airport's flight capacity by 10 per cent.

Stansted divisional police commander Ian Gruneberg told Sky News it was a concern the protestors had been able to penetrate the airport's security by cutting through perimeter fencing.

"It is a matter we will be reviewing with the airport later to see what we can learn from it to see if we can make it more difficult next time," he said.

The airport said it would investigate the security breach.

"Where there are lessons to learn we will do so," it said.

Protestor Lily Kember told Sky News from the runway before her arrest the group's action would stop tonnes of carbon dioxide being released into the atmosphere.

"Climate change is going to affect everybody, it's a global catastrophe in the making and we need to take action now," she said.