Heathrow could be given the go-ahead to build a new runway by Prime Minister David Cameron before the end of the year, in a move which the airport and its supporters say will add new flights and boost the economy.
But some local residents and climate change groups say expansion will bring additional noise and air pollution.
Heathrow said on Thursday that traffic was affected, while Transport for London warned of queues and delays in the area.
"We advise passengers to allow extra time getting to the airport today. Our priority is to return to normal operations as quickly as possible," Heathrow said in a statement.
Police said that they had now finished dealing with the protest.
The airport, owned by Ferrovial, Qatar Holding, China Investment Corp and others, was forced to cancel over a dozen flights in July when activists from the Plane Stupid group chained themselves together on the runway.
Claiming responsibility for Thursday`s incident, Plane Stupid said on their website that three of its members had parked a vehicle across the tunnel and locked their bodies to it.
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