* Activists cut through fence, chain themselves together

* Protest against possible airport expansion

* Heathrow forced to cancel some flights

* Police arrest nine 

LONDON: 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 ($36 billion) 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.

The size of the runway meant most planes were able to take off during the protest. 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 and a maze of hangars.

HEATHROW BATTLE?

The disruption indicates the potential challenge the airport will face if Prime Minister David Cameron decides to grant Heathrow a third runway, a decision he is due to take by year end.

Heathrow was selected as the site for a new runway in south east England earlier this month and ministers say Britain needs at least one new runway if London is to remain the dominant city in its timezone.

For decades, politicians have discussed where to build the runway but have shied away from a final decision that they expect will provoke a vociferous campaign by environmentalists and residents.

The runway "was accessed through the fence. It took a matter of minutes," Joshua Virasami, an activist who said he was involved in the planning of the protest, told Reuters.

"No if, no buts, airport expansion will not happen, and we will ensure that," he said. Another activist told Reuters that no guards were around when they cut through the fence.

Previous plans to build a new runway at Heathrow were scrapped in 2010, in part due to environmental concerns.

Heathrow's largest shareholder is Spanish infrastructure firm Ferrovial. Other partners include Qatar Holding, China Investment Corp and the Government of Singapore Investment Corp.