Whitehall sources say he has resisted pressure from the environment secretary, Margaret Beckett, to tone down his plan, which he regards as the most important decision he will take as a transport minister.
He has stopped short of the most controversial option, building a third runway at Heathrow, the country's busiest airport. But he is likely to offer a review of the rule prohibiting the simultaneous use of both existing runways.
To the dismay of environmentalists, there will be no increase in the air passenger duty levied on tickets. But Mr Darling may favour a scheme intended to make airlines pay for the cost of their pollution.
Protesters near Stansted, backed by Essex county council, are preparing to sue the government on the grounds that it has paid insufficient attention to the environmental consequences of a new runway.
Carol Barbone, director of the Stop Stansted Expansion campaign, vowed to block the extra runway at a public inquiry.
Mr Darling's announcement will end 18 months of uncertainty since the publication of a consultation paper. It will finally rule out new international airports near Rugby and at Cliffe in north Kent. Birmingham airport will be favoured for expansion over East Midlands airport. There will also be a long-term decision on airport expansion in Scotland, between Glasgow and Edinburgh.
Airlines may take legal action, led by British Airways and Virgin Atlantic, which say they will challenge any attempt to fund a new runway at Stansted by increasing landing charges at Heathrow. The BA chief executive, Rod Eddington, has warned that unless Heathrow gets an extra runaway, British aviation could go into a spiral of descent akin to shipbuilding and coalmining.
In an attempt to dampen their ire, the government will propose ending the restrictions which require Heathrow to alternate its two runways each day, using one for take-offs and one for landings to relieve the noise nuisance.
A review next year will consider full use of both, which could increase the annual capacity from 65 million passengers to 105 million. That is likely to enrage local communities. They say they will never have any relief from noise. Critics say it will complicate air traffic control.
BA said: "It's a short-term fix which will address some of the capacity constraints. But the case for a third runway remains compelling."
The government regards airport expansion as essential to cope with the growing appetite for cheap foreign travel. The number of people passing through the country's terminals rose from 30 million in 1970 to 180 million in 2000, and is forecast to reach 500 million by 2030.
Environmentalists say aviation benefits from £9bn of tax advantages annually. Transport 2000 has suggested that raising air passenger duty, introducing fuel tax, and putting VAT on tickets would squash demand and avoid the need for expansion.
Locals held a procession at Heathrow on Saturday, ending with a carol service in a 15th-century tithe barn which would be demolished if a new runway were to be built.
