A £14bn “masterplan” to expand Heathrow by diverting rivers, demolishing hundreds of houses and rerouting a motorway beneath a third runway has been met with public silence by Boris Johnson, who has been a longstanding opponent to expansion.

Campaigners say the proposals, which come just weeks after the government declared a climate emergency, will be devastating to local communities and impede Britain’s goal of drastically reducing carbon emissions.

Under plans made public on Tuesday, expansion will see flights increase from 480,000 a year to 756,000, and passenger journeys up from 80 million to 142 million, resulting in millions of tons of additional carbon emissions each year.

To meet the extraordinary surge in capacity, a new runway, which would be completed by 2026, will be built over a specially constructed tunnel through which the M25 will be diverted. The rest of the airport infrastructure, including terminals, roads and a giant car park near Sipson village, will be completed by 2050.

But Mr Johnson, the overwhelming favourite to be Britain’s next prime minister, has been conspicuously silent on the issue in recent months.

In 2015 he told local residents he would lie down in front of bulldozers to prevent expansion of the airport, which sits next door to his Uxbridge and South Ruislip constituency.

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On Tuesday, however, his office failed to respond to questions about whether as leader he would ditch the plans. It comes a year after he took a much-mocked flight to Afghanistan as foreign secretary in a ploy to avoid voting on the proposals.

Mr Johnson did, however, last week hint during a hustings organised by Conservative backbenchers that he would allow expansion to go ahead. Asked if he would cancel the scheme should he become prime minister, he said it had already been approved by parliament, according to The Times.

Sian Berry, co-leader of the Green Party, told The Independent the “only answer” to Heathrow expansion in light of Britain’s climate emergency is “a big fat no” to the plan.

Artist’s impression of Heathrow’s expansion plans (Grimshaw Architects)

“Every MP who voted to allow us to get to this consultation should take a long hard look at themselves. Aviation clearly has to contract, not expand, while we need to promote and encourage cleaner options like train travel, which could replace many Heathrow flights.

“I’d urge Londoners in particular to tell Heathrow exactly what they think about the plan, to express their feelings also about the impact it would have on transport capacity, air quality and mental health and wellbeing.”

John Sauven, Greenpeace UK executive director, said: “Every time Heathrow has expanded, they have sincerely, faithfully and passionately promised that they will never expand again. In 1999, they wrote to local residents promising to lobby the government to make building a third runway there illegal.

“Now they say they can build a third runway which will double the number of flights without increasing road traffic, air pollution, carbon emissions or noise.

“Alongside this they want to expand the rail network and bury the M25, all with no discernible source of funding. And hapless transport minister Chris Grayling apparently believed them. This whole absurd situation would almost be funny if we weren’t facing a climate emergency.”

Stop Heathrow Expansion, a pressure group, said the plans were “laughable”.

“Not only does it want to disrupt people’s lives for up to 30 years whilst building this new runway but now proposes jumbo-size car parks whilst pledging to reduce the number of people using cars at the airport,” a spokesperson said.

“How can anyone believe any of their pledges, and more worryingly, who is there to hold them to account when their promises are broken?”

Heathrow insisted it would focus on mitigating the environmental impact by creating low-emission zones and congestion charges to limit local air pollution, while providing noise-insulation funding for local residents and imposing a 6.5-hour ban on scheduled night flights.

It also claimed there would be no more cars using the airport after expansion, despite plans to build another car park.

Emma Gilthorpe, Heathrow’s executive director for expansion, said: “Expansion must not come at any cost. That is why we have been working with partners at the airport, in local communities and in government to ensure our plans show how we can grow sustainably and responsibly – with environmental considerations at the heart of expansion.”

Heathrow’s final plans, which will include public responses, will be put to planning inspectors next year before being put to the transport secretary for final approval.

It comes amid an unprecedented global warming crisis, which the United Nations last month said in a new report is putting Earth’s “very survival” in doubt.