By Sarah Young
LONDON, Dec 6 (Reuters) - Britain's Prime Minister David Cameron could approve as soon as this week the building of a new runway at Heathrow Airport, ending decades of political deadlock as well as breaking a famous pledge made in 2009 not to allow a third runway to be built at Europe's busiest hub airport.
The debate over where to build a new runway in the densely populated southeast of the country has been raging for over 25 years and in 2010 permission was withdrawn to expand Heathrow, which objectors say already blights the lives of hundreds of thousands of Londoners.
But five years on and following the recommendation of the independent Airports Commission, Cameron is now widely expected by British media to give the green light to a revised 23 billion-pound ($35 billion) plan to give Heathrow a third runway.
That would see him renege on the pledge he made to voters before the previous election in 2010 that Heathrow would not get a third runway under his watch, "no ifs, no buts".
But in July the Airports Commission, which was appointed three years ago by Cameron's previous coalition government to assess how and where to add new runway capacity, recommended Heathrow's expansion as the best of all the options.
In response Cameron said he would make a final decision before the end of 2015. His spokesman confirmed on Friday the government would respond to the Commission's recommendation by the year end.
But he faces a tricky time winning any future parliamentary vote on the new runway given resistance among British lawmakers both from within his own Conservative party and in the opposition Labour party, whose leader and finance minister oppose it.
London Mayor Boris Johnson, a Conservative member of parliament, has said he would lie down in front of bulldozers to stop the third runway going ahead, while his potential successor as mayor, Zac Goldsmith, another Conservative MP, is also a prominent opponent to Heathrow's expansion.
Meanwhile campaign group Plane Stupid, which is opposed to any expansion of air traffic in the UK, has already sought to disrupt Heathrow's operations in protest against the proposed new runway.
In July, activists cut through Heathrow's perimeter fence and chained themselves on a runway, forcing some flights to be cancelled. Last month they brought chaos to road traffic around the airport by blocking the main entrance tunnel with a vehicle.
It is expected that any approval for Heathrow from the government will place a strong emphasis on the conditions set by the Commission to address environmental concerns.
These included a ban on night flights between 2330 and 0600, legally binding limits on the level of noise created by the airport and a legal commitment on air quality.
A separate campaign group, Stop Heathrow Expansion, remains opposed to the plans, arguing that air pollution levels in some places close to Heathrow are already above the legal EU limits.
Heathrow's chief executive told lawmakers in November that he was confident that an expanded Heathrow would be able to meet EU air quality limits, promising that the airport would only release capacity from the new runway if it was clear that doing so would not delay compliance with those limits.
Heathrow, which is operating at full capacity, argues that a new runway will improve Britain's links to other markets, adding 100 billion pounds to the economy and more than 120,000 new jobs.
The new runway at Heathrow was preferred by the Commission to two other options which it had shortlisted - the extension of Heathrow's existing northern runway and building a second runway at London Gatwick - with the Commission saying Heathrow's third runway offered the best way to add "urgently required" long-haul routes to new markets and would provide the most benefits to the wider economy.
Heathrow's largest shareholder is Spanish infrastructure firm Ferrovial. Other partners include Qatar Holding, China Investment Corp and the Government of Singapore Investment Corp. ($1 = 0.6610 pounds) (Additional reporting by William James; Editing by Greg Mahlich)
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