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.

“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 in a statement. “That’s why we’re here.”

Europe’s busiest hub said it’s working closely with police dealing with the incident, and that passengers should check the status of their flight before traveling to the terminal.

The incursion is also an embarrassment for Heathrow and security forces at a time of heightened concern about the potential for attacks by supporters of Islamic State or other extremists. Some 30 Britons were killed June 26 in a shooting at a holiday resort in Tunisia, Britain’s highest death-toll in a terror incident since the July 7, 2005, bombings in London.

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