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LONDON (AP) — Don't expect the congestion in the skies above Heathrow Airport to magically clear up anytime soon.

The British government on Thursday approved the construction of a controversial third runway for Heathrow — Europe's busiest airport — but that doesn't mean it will be built in the next year or two.

Transport Secretary Geoff Hoon told Parliament the new runway and a new passenger terminal should be ready between 2015 and 2020. But that optimistic timetable may be delayed if a bevy of determined opponents can slow the permit process with legal challenges.

There is another X factor: it is not clear the runway project can survive if the Conservative Party, which is opposed to the scheme, wins the next election, which must be held by the middle of next year.

National polls have long been predicting a Conservative Party victory — leading opponents of the runway to devise a strategy based on stalling.

"Despite this decision, all is far from lost," said Zac Goldsmith, environmental adviser to Conservative Party leader David Cameron. "If the Conservatives maintain a good lead in the polls, and if campaigners can slow the process down by a few more months, the battle against the Third Runway will still be won."

Officials at British Airways and Virgin Atlantic hailed the decision to build the 9 billion pound ($13.1 billion) runway, and union leaders were pleased with the estimated 65,000 jobs to be created.

The government's decision is designed to keep Heathrow, which now has only two runways, competitive with other major European airports, including Amsterdam's Schipol, with five runways, and Paris Charles de Gaulle, with four.

Heathrow handles more than 450,000 flights a year, including many lucrative cross-Atlantic routes.

In an effort to mollify the powerful environmental lobby, Prime Minister Gordon Brown's government made a number of concessions.

Hoon announced that billions of pounds (dollars) would be spent on new high speed rail links to and from Heathrow and a new high speed railway linking London to northern England. He said large parts of Britain's rail network would be electrified so quieter, cleaner trains could be used.

And he said European Union noise pollution limits would be met at Heathrow, despite the new runway, because only environmentally advanced planes would be allowed to use it. He also said the government will only allow 125,000 more flights per year, not the 220,000 that had been sought.

Hoon said these changes would allow Britain to lead the fight against climate change while still adding airport capacity. The British government is committed to cutting carbon emissions 80 percent by 2050.

"Taken together, this gives us the toughest climate change regime for aviation of any country in the world," Hoon said.

Still the debate became so heated Thursday that Labour lawmaker John McDonnell was suspected for five days after picking up the ceremonial Parliamentary mace in a threatening matter.

But activists were not convinced, saying the plan would require destroying 700 homes and significantly increase noise levels for 2 million people nearby.

"People will fight the government in the courts, in their communities, in the town halls, and if necessary on the tarmac of Heathrow," said John Stewart, leader of a local opposition group.

The environmental coalition against the runway includes Greenpeace, which has an extensive legal budget, and key political leaders including Cameron; Nick Clegg, leader of Britain's third largest party the Liberal Democrats; and London Mayor Boris Johnson — who favors building a new airport east of London in the Thames River estuary.

Johnson said Thursday that Heathrow should never have been built in its current location and that he would fight its expansion.

"No amount of sweeteners in the shape of transport infrastructure will fundamentally alter the fact that the government is hell-bent on exacerbating a planning error of the 1940s," Johnson said.

Celebrities, including Oscar-winning actress Emma Thompson and comedian Alistair McGowan, helped buy a one-acre plot of land where the runway is supposed to be built to serve as a focal point for public protests.