LONDON, June 6 (Reuters) - British lawmakers could approve the expansion of Heathrow this month, moving London's largest airport closer to building a new runway than it has been for years, but hurdles remain after decades of indecision and false starts.
Owned by Spain's Ferrovial, Qatar Investment Authority and China Investment Corporation among others, Heathrow is Europe's busiest airport but is now operating at full capacity.
Below is a look at what has happened in the past and why the latest 14 billion pound ($18.8 billion) plan could have more chance of success than its predecessors given that new planning laws mean the principle of expansion cannot be challenged.
Lawmakers will vote on the issue within the next few weeks, making the start of work in 2021 a possibility. Heathrow says the runway would be operational by 2026.
In 1968, the British government set up a commission to consider the need for new airport capacity in the south east of England. Fifty years later and London is still waiting for a new full-length runway.
Heathrow last came this close to building one in 2009, when the government granted it approval to expand, but an election the following year ushered in a new government which blocked development of the new runway and it abandoned the plan.
In 2012, under pressure from business leaders and airlines, the government set up the Airports Commission, an independent inquiry to finally decide what to do. Three years later, it picked Heathrow over London's number two airport Gatwick.
The Airports Commission's three years and 342 pages of research and analysis said building a new runway at Heathrow would bring the most economic benefits to the country.
Britain voted to leave the European Union in 2016, which many politicians and business groups agreed strengthened the need to expand Heathrow to serve new trade links to other parts of the world.
The last time Heathrow, which currently has two runways, expanded was in 2008, when it opened Terminal 5. That project was granted planning permission eight years after it was requested following an inquiry that sat for almost four years.
Britain's planning laws changed in 2008 to try to accelerate the process for nationally significant infrastructure projects.
Under those rules, the government sets out a National Policy Statement regarding airport expansion and the planning inspectorate will consider Heathrow's planning application over a set period of around 18 months, on how well it matches the NPS.
"In terms of the principle of a new runway in the south east of England and Heathrow and the new north west runway, that is settled by the National Policy Statement and it is not possible to re-open that issue of principle during the planning process," said Robbie Owen, head of infrastructure planning at Pinsent Masons, and an external legal advisor to Heathrow.
"That essentially is why there is a much greater certainty this time round."
The lawyer said that Heathrow has a lot of work to do to put together its planning application, which will include it proving how expansion, which will add more polluting planes and vehicles, can be done without breaching air quality rules.
"It's not going to be an easy ride because it's a complex scheme but the process is working well and with the clear guidance we now have through the NPS there's no reason why the process can't be completed," Owen said.
While the anti-Heathrow expansion group HACAN remains opposed to the new runway, its chairman said that the latest plans are an improvement on what was on offer in 2009.
"One of the differences from last time is that, the third runway last time, local people weren't offered anything at all," Chairman John Stewart said.
This time, the new runway is accompanied by a number of conditions governing expansion, such as a 6.5-hour night flight ban, compared to a current restriction on the number of flights during night time hours.
Moreover the airport is pumping 2.6 billion pounds into providing compensation for local people, a figure which the transport minister said was 10 times bigger than under the 2009 third runway proposal.
But before it can get to the planning stage, the new runway at Heathrow may face a legal challenge from a judicial review, where a judge reviews the lawfulness of the government's policy.
"If Heathrow falls over it'll be on account of air quality because air quality is in a sense beyond Heathrow's control," Al Watson, head of planning and environment at law firm Taylor Wessing said.
Legal rules govern air quality standards and the air quality around Heathrow is known to be low given the airport's proximity to two motorways.
Four local councils and environmental campaign group Greenpeace have six weeks from the date of the vote in the House of Commons to make the case for a judicial review, something they tried last year but were told to wait until the NPS was published.
First a judge must agree that there are grounds for a judicial review.
Should a review go ahead, a judge could then look to strike out an offending part of the NPS or order the government to change, review or scrap its policy. The court cannot directly block the new runway from going ahead.
Polling shows that a majority of lawmakers will vote in favour of the new runway, but the vote could be tighter than expected. A Labour spokesman suggested on Wednesday that the party could vote against it, potentially joining forces with a number of high-profile Conservative rebels.
The expansion plan could also be derailed if there is a snap election and a Labour government replaced the Conservatives as shadow finance minister John McDonnell is a long term opponent of the third runway.
Any delays to the planning process could also risk Heathrow's chances of success as the next election is scheduled for 2022. Heathrow has said that building will be start in 2021 but any delays could give a new government a chance to stop the process. ($1 = 0.7447 pounds) (Reporting by Sarah Young Editing by Keith Weir)