"There is a protest taking place in the inbound tunnel which is currently affecting traffic around the central terminal area and terminals 2 and 3 at Heathrow," a Heathrow spokesman said.
"Police are on site but we advise passengers to allow extra time getting to the airport today."
Drivers arriving at the airport were being re-routed to avoid the protesters and passengers told to go to other terminals.
"Airport expansion would wreck the legally-binding Climate Change Act," said Cameron Kaye, a spokesman for environmental campaign group Plane Stupid.
"The government needs to choose: build new runways or stop climate chaos: it`s that simple," he added.
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