Protesters block tunnel at London's Heathrow airport

LONDON, Nov 26 (Reuters) - 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. (Reporting by Sarah Young, editing by James Davey)