Sir Sadiq Khan has threatened legal action against the government over its controversial plans to allow a third runway at Heathrow.
The London mayor doubled down on his opposition to the plans, warning that it will have a “severe impact” in terms of noise and air pollution and leave Britain’s climate targets in jeopardy.
But the chancellor Rachel Reeves said it was "essential" that the UK increases its airport capacity.
Sir Sadiq said he will “carefully scrutinise” Heathrow’s expansion plans and the impact they will have on Londoners.
And, raising the prospect he could sue Sir Keir Starmer’s government, the London mayor said: “I’ll be keeping all options on the table in how we respond.”
He added: “I remain opposed to a new runway at Heathrow airport because of the severe impact it will have in terms of noise, air pollution and meeting our climate change targets. I remain unconvinced that you can have a new runway at Heathrow, delivering hundreds of thousands of additional flights every year, without a hugely detrimental impact on our environment.”
His comments came as Heathrow said it could build a third runway for £21 billion within a decade. It attributed the increase from its estimate of £14 billion in 2018 to "construction inflation".
The total plan, including terminals and supporting infrastructure, would be expected to cost £49 billion.
Heathrow believes it is possible to meet the Government's ambition of securing planning consent by 2029 and the new runway being operational within a decade.
But Sir Sadiq warned of a “huge knock-on effect” on London’s transport infrastructure if a third runway goes ahead, warning that plans would need to be implemented to manage the impact.
Rachel Reeves has already said a challenge to the expansion plans, part of her and Sir Keir’s push to get the economy growing, would fail.
The defiant chancellor said she has “huge respect” for Sir Sadiq, who is one of the most influential figures in the Labour Party, but she said she disagrees with the London mayor and is confident the third runway will go ahead.
On a visit to Scotland on Friday, she said: "It is essential that we increase airport capacity in the UK.
"Two of the biggest exports from Heathrow are Scottish salmon and Scotch whisky, so this is good for the whole United Kingdom, absolutely essential to ensure the things that we make here in Britain can be exported and sold throughout the world.
“And the applications that have come in will include investment in the infrastructure, for example, the changes around the M25 to make airport expansion necessary.
“But airport expansion is essential if we're going to seize the opportunities, and this Government are absolutely determined to seize the opportunities that come from being an open trading economy."
Sir Sadiq’s fresh opposition to the runway plans came as transport secretary Heidi Alexander welcomed Heathrow’s proposals for its development.
Ms Alexander said: “I am pleased to have received the initial Heathrow expansion proposals – a significant step towards unlocking growth, creating jobs, and delivering vital national infrastructure.”
She said the government will consider the proposals over the summer before a final decision on whether to give it the go-ahead.
After months of flatlining growth since Labour came to power, Ms Reeves in January unveiled the plans for a third runway at Heathrow to boost the economy.
She said pro-growth measures must trump other priorities such as the green agenda. But the plans have also sparked opposition from Andy Burnham, another senior Labour mayor. The Greater Manchester mayor has warned the third runway will concentrate economic growth in London.
Energy secretary Ed Miliband has long opposed a third runway, while Sir Keir himself has previously voted against the airport’s expansion.
Britain’s second-busiest airport, London Gatwick, has also been given a conditional go-ahead for its expansion plans as part of the government’s push for growth.
It intends to increase traffic on what is already Europe’s busiest single runway, and reconfigure the standby runway for routine use by departing aircraft.
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