LONDON, July 13

Heathrow Airport was forced to cancel 13 flights on Monday after activists cut through a perimeter fence and chained themselves together on a runway to protest against the possible expansion of Britain’s biggest airport.

In a stunt which raised questions about the security of Europe’s busiest airport, a group of at least 12 activists said it took only minutes to get through the apparently unguarded wire fence at around 0230 GMT allowing them access to the northern runway.

The protest was eventually ended by police more than seven hours later, the airport said.

The activists, from a group called ‘Plane Stupid’ which opposes the growth of aviation, said a 23 billion pound plan to build a third runway at Heathrow would increase carbon emissions and would trigger further protests.

“These individuals have since been removed by police,” Heathrow Airport said in a statement. “Both runways were operational throughout, although there have been some delays and a few cancellations.”

Pictures posted by activists on Twitter showed them chained together, some smiling under blankets at dawn and lying beside the black scars left by the tyres of jets. Later pictures showed some being led away by British police.

Although frustrating for business and summer holiday travellers, the 13 cancellations are a small part of the average 1,290 daily landings and departures at Heathrow.

Police said they had made nine arrests.

At a time when Britain is on its second highest alert level of “severe” — meaning a militant attack is considered highly likely — the incursion on to the runway also underscored the challenge in guarding an airport with miles of fences.