Activists opposed to the expansion of London's Heathrow Airport blocked an approach tunnel on Thursday by chaining themselves to a parked vehicle, sowing traffic chaos around Europe's busiest airport.
The airport said police were dealing with a protest in a tunnel used by cars and taxis to access its terminals.
"We can confirm there is a protest taking place in the inbound tunnel which is currently affecting traffic around the central terminal area," Heathrow said in a statement.
Prime Minister David Cameron is due to make a decision before the end of the year on whether to proceed with a recommendation to build a new runway at Heathrow, a move which would disappoint some local residents and climate change groups who oppose airport expansion.
Protest group Plane Stupid said on their website that three of its members parked a vehicle across the tunnel and locked their bodies to it.
In July, activists from the same group delayed flights when they chained themselves together on the runway.