Protesters are staging a demonstration at Heathrow airport to oppose plans for a third runway at the UK's biggest airport.
Three activists, part of the Rising Up! group, claim to have chained themselves to a vehicle and police confirmed officers are at the scene.
"Heathrow airport can confirm that a protest activity is on-going in the in-bound tunnel to Terminals 2 and 3," said a Heathrow spokesman.
"Heathrow supports the right to peaceful protest within the law, but the safety and security of our passengers, aircraft and colleagues together with the smooth running of the operation is paramount.
"Contraflow traffic measures are in place and we are working closely with the authorities to resolve the issue, but advise passengers to allow more time to drive into Terminals 2 and 3."
One of those involved in the protest described a consultation process for the proposal for airport expansion as "illegitimate and undemocratic".
"This blockade is our contribution to the consultation: a third runway is a disastrous option that will lead to climate chaos," said Simon Bramwell.
"I look upon my niece and nephew, upon the children who will inherit the future we are creating right now and my conscience tells me to act.
"I am breaking conditions imposed on me by the courts, following road blockades in November but the Government is not listening to the science or to our concerns. They have left us with no alternative but to keep taking action.
"We will not stop until plans to build another runway are fully and finally shut down."