Climate change activists have blocked the inbound tunnel to Heathrow in a protest against airport expansion.
Three members of campaign group Plane Stupid parked a vehicle across the lanes of the main road into Heathrow and locked themselves to it.
They displayed a banner with the message: "No ifs, no buts: no third runway."
Police were called to the scene at 7.42am and no arrests have been made, a Scotland Yard spokesman said.
Plane Stupid spokesman Cameron Kaye said: "Airport expansion would wreck the legally-binding Climate Change Act, risking wiping out 55% of species this century and displacing 75 million more people from their homes by 2035.
"If aviation growth isn't reduced, by 2037 aviation alone could emit all of the carbon it's safe for the UK to emit. The government needs to choose: build new runways or stop climate chaos: it's that simple."