LONDON - London's Heathrow airport suffered a major security breach as climate-change protesters opposed to the hub's expansion plans penetrated the perimeter fence, causing the cancellation of at least 21 flights.

A dozen activists cut through the wire at 3:30 a.m. Monday before chaining themselves together on Heathrow's northern landing strip, according to campaign group Plane Stupid. The incursion comes less than two weeks after a government-backed study named Heathrow as the best option for a new London runway.

Heathrow was finally operating normally late Monday morning after the northern strip was closed and then limited to short-haul jets. Monday's protest is a warning shot for Prime Minister David Cameron, who must decide whether to back the Airport Commission findings and expand Heathrow — most likely sparking off years of protests by environmental groups and political opponents.

"This would massively increase carbon emissions, exactly when we need to massively reduce them," Plane Stupid activist Ella Gilbert, who took part in the runway invasion, said of the Heathrow growth plan. "That's why we're here."

The incursion is an embarrassment for Heathrow and security forces at a time of heightened concern about the potential for attacks by supporters of Islamic State in Iraq and the Levant or other ­terrorists.

"The safety of our passengers and the smooth running of the operation is paramount," Heathrow said in a statement, promising a full investigation of the perimeter penetration. "Anyone who breaks the law and interferes with the safe operation of the airport can expect to face legal action."

Heathrow should get a third runway by 2026, the Airport Commission said July 1, with the $28 billion project lifting passenger numbers above 135 million by 2050, creating 70,000 jobs and adding 40 destinations,

Anti-Heathrow campaigns led by London Mayor Boris Johnson and Conservative lawmaker Zac Goldsmith have highlighted a likely gain in noise levels and particulate emissions to argue that a new runway should be built in a less populated area.