The Government's long-awaited decision on which airport scheme should get the go-ahead will finally be announced later.
A third runway at Heathrow is expected to be announced.
But other options include extending an existing runway at Heathrow or building a second runway at Gatwick.
Transport Secretary Chris Grayling will make the announcement in the House of Commons at around 12.30pm.
Downing Street said any ministers wanting to voice their opposition to the plans would have to seek permission from the Prime Minister in advance.
Following the announcement, a public consultation will be held on the effects of airport expansion.
Then a final decision by the Government, which will be part of a National Policy Statement on Aviation, will be made.
This will be put to MPs for a vote in the winter of 2017/18.
A number of groups have come out in support of Heathrow, including devolved governments, business leaders and politicians.
But Gatwick insists it is the only project that can be delivered, citing concern about the noise and environmental impact of Heathrow expansion.
Gatwick chairman Sir Roy McNulty issued a last-minute plea, saying allowing London to have "two world-class airports" would send a powerful signal to the world that "Britain is truly open for business".
He claimed the environmental impacts of expanding the West Sussex airport would be "a fraction of those at Heathrow", adding: "Gatwick represents the best chance of something actually happening. It is the deliverable option."
Anti-expansion groups are gathering in Harmondsworth, near Heathrow, and Crawley, near Gatwick, ahead of the announcement.
Environmentalists concerned about the effect of flying on climate change are holding a demonstration outside the House of Parliament on Tuesday morning.