Activists from both sides of the Heathrow debate are set to clash in rival demonstrations on Thursday.
In the green corner, eco-warriors from Campaign Against Climate Change (CACC) will show their opposition to a third runway by donning Gordon Brown masks outside Downing Street.
Meanwhile, members of pro-expansion group Modern Movement will gather in nearby Parliament Square to make the case for 'faster, cheaper' transport.
Both demos will begin at 5.30pm and are expected to last for about two hours.
Speakers at the CACC event will include firebrand MP John Donnell, who leapt to fame by wielding the mace during a recent Commons debate, and Richmond Park MP Susan Kramer.
Campaigners, who have been asked to dress in brown and green, will unfurl banners with messages including 'climate suicide' and 'business as usual'.
The organisers of Modern Movement's counter-demonstration claim a third runway will make air travel more affordable, and open up new destinations, for millions of Britons.