The police have warned climate protesters they will face possible life sentences if they fly drones around Heathrow Airport.
Extinction Rebellion last month threatened to shut down the airport with drones on 18 June, in protest against a planned expansion.
In a strongly-worded statement issued on Friday afternoon, Scotland Yard urged campaigners to reconsider.
Extinction Rebellion declined to comment.
Deputy Assistant Metropolitan Police Commissioner Laurence Taylor warned the environmental protesters against doing anything to disrupt or endanger the UK's most important airport.
'Disruption and misery'
He said: "If flown into the path of an aircraft, a drone has the potential to cause great harm to those on board.
"Endangering the safety of an aircraft can result in a life sentence.
"We would urge anybody intending to join this event with a view to committing criminal activity, whether considered peaceful or not, to strongly reconsider.
"The airport is part of our national infrastructure, and we will not allow the illegal activity of protesters to cause disruption and misery to thousands."
Commanders say they have devised a policing plan to guard the airport and surrounding area and in order to make it work they have diverted officers from frontline roles elsewhere in London.
Extinction Rebellion said it would release a statement on Sunday.
A spokesperson for Heathrow Airport said: "We are working with the authorities to address any threat of protests which could disrupt the airport.
"This is reckless action that if carried out could endanger the lives of the travelling public and our colleagues.
"We agree with the need to act on climate change, but that requires us to work together constructively - not commit serious criminal offences."
Departures at Heathrow were temporarily stopped in January after a drone was reported to have been sighted.
In December 2018, a "sustained" drone attack caused disruption for tens of thousands of passengers at Gatwick Airport, after more than 100 drone sightings over three days.
Following the chaos, the government extended the no-fly zone for drones around airports.