The UK is backing a $US22 billion ($A29 billion) expansion of Heathrow Airport, ending 25 years of indecision and promising to boost global trade links following the vote to leave the European Union.

A British Airways 747 flies over London rooftops as it comes in to land at Heathrow, the world’s busiest for international passenger traffic. Photo: Getty

Heathrow, Europe’s busiest airport, defeated a proposal from smaller rival Gatwick and will secure the first new full-length runway to be built near London in 70 years after environmental and political protests scuppered previous attempts.

Prime Minister Theresa May, a former critic of Heathrow expansion, said she had opted for a plan that would boost the economy, create jobs and provide access to global markets.

Airport protest group Stop Heathrow Expansion tweeted over her change in heart, pointing out that in 2010 she welcomed the cancellation of the third runway. Others also shone a light on the u–turn.

"Nobody will take this Government seriously on the environment again" ~ Theresa May, on Labour plans for a third runway at #Heathrow, 2009.

— Ben Cobley (@bencobley) October 25, 2016

Her decision put her on an immediate collision course with senior politicians, including Foreign Secretary Boris Johnson, who opposes the plan.

The decision to jump-start one of Europe’s biggest infrastructure projects is one of May’s most significant moves since she took office in July and follows her approval in September of a $US24 billion nuclear power plant at Hinkley Point.

By choosing to build a third runway at Heathrow, which will require rebuilding parts of the motorway circling the capital, May opted for the more expensive and complex option over cheaper plans to extend an existing Heathrow runway or build a new one at Gatwick, south of London.

According to a three-year study by Britain’s independent Airports Commission, a new runway at Heathrow would create 70,000 new jobs by 2050 and increase gross domestic product by between 0.65 and 0.75 per cent over the same period.

It will also enable Britain to keep pace with Europe’s biggest airports in Paris, Amsterdam and Frankfurt, which boast greater capacity, while Heathrow’s established links with emerging markets were seen to strengthen its case in the wake of Britain’s June vote to leave the EU.

Protestors from The Plane Stupid action group under blankets after occupying party of Heathrow’s northern runway in 2015. Photo: Getty

Virgin Group founder Sir Richard Branson said the move would ensure Britain did not lose its place to Paris or Berlin, and would generate tens of thousands of jobs.

He said it was a brave decision by the government, but in 10 years time planes would be a lot quieter than they are now.

But within hours of the decision, politicians were lining up to denounce it.

Johnson said a third runway was “undeliverable” and “very likely to be stopped” while London Mayor Sadiq Khan said he would continue to oppose Heathrow’s expansion.

Protesters marched against expansion plans in 2008, angry about both the effect on locals and the environmental aspects of the move. Photo: Getty

Zac Goldsmith, a lawmaker in May’s Conservative Party, immediately resigned over the issue, causing a by–election in his constituency near the airport and potentially threatening May’s already slim majority in parliament.

Adam Marshall, director general of the British Chambers of Commerce, said governments had prevaricated for too long.

Heathrow is owned by Spanish infrastructure company Ferrovial, Qatar Holding, China Investment Corp and other investors, and the bill for expansion will be paid for by the private sector, with the government expected to pay for some of the additional road and rail costs.

Engineering firms Arup and US-headquartered CH2M, British construction company MACE and construction and project management firm Turner & Townsend are already working on the early stages of the project.

Government backs 3rd runway at Heathrow. Oh deary me. https://t.co/I4bMYq6X7O

— Jason Sayer (@adjasoncies) October 25, 2016

The government also proposed legally binding noise targets to provide respite for local residents and Heathrow will also need to meet air quality rules as a condition of planning approval.

– Kylie MacLellan and Sarah Young