Passengers are being warned to expect delays at Heathrow today after climate change activists cut through a fence and chained themselves together on the northern runway.
Members of action group Plane Stupid, who are protesting against airport expansion, entered the runway at 03:30am.
Heathrow said they are working with the police to resolve the incident but said there would be delays.
"Both runways are open although there will still be delays - we are sorry for the disruption to passengers. Our priority remains to ensure the safe running of the airport," Heathrow said in a statement.
Passengers have been told to check with their airlines before they travel.
The southern runway remains open.
Speaking to the BBC, Ella Gilbert, one of the activists on the runway, said: "We want to say sorry to anyone whose day we've ruined, and we're not saying that everybody who wants to fly is a bad person.
"It's those who fly frequently and unnecessarily who are driving the need for expansion, and we cannot keep ignoring the terrifying consequences of flying like there's no tomorrow."
Earlier this month, the Airports Commission inquiry into airport expansion in south-east England recommended a third runway at LHR.