EXPANDING Heathrow airport may spark Britain’s biggest-ever single environmental campaign, former shadow chancellor John McDonnell warned today.

The MP for Hayes and Harlington, which contains Heathrow, was commenting as a new expert report rubbished claims that a third runway would deliver economic growth amid reports that the Chancellor is preparing to back the project.

Mr McDonnell said: “Backing a third runway at Heathrow would shred the government’s credibility in one fell swoop and potentially launch the greatest single environmental campaign this country has seen.

“I can’t believe that the Cabinet would make this mistake and put so many [parliamentary] seats at risk.”

Ms Reeves is reportedly expected to use a speech on growth next week to support the proposed runway and endorse expansion at Gatwick and Luton airports.

Alethea Warrington of climate charity Possible said approving the expansions would be “a catastrophic misstep for a government which claims to be a climate leader” and would only help frequent flyers.

Friends of the Earth transport campaigner Jenny Bates said: “A decision to green-light another runway at Heathrow would be hugely irresponsible in the midst of a climate emergency.

“[It] would also fly in the face of the Prime Minister’s promise to show international leadership on climate change.”

Heathrow’s third runway project secured parliamentary approval in June 2018, but has faced legal delays and pandemic disruption.

Transport Secretary Heidi Alexander has until February 27 to decide whether to allow Gatwick to bring its existing emergency northern runway into routine use, and April 3 to decide on Luton airport’s bid to raise its cap on passenger numbers.

No 3rd Runway Coalition chairman Paul McGuinness said Gatwick and Luton’s expansion projects are “oven ready” whereas an application by Heathrow would take years to process.

GMB national secretary Andy Prendergast said a green light is long overdue, adding: “Expanded capacity at the airport is crucial for our country’s prosperity and has been put off for far too long. 

“The extra runway is crucial to create thousands of good, union jobs. The sooner the project is progressed, the better.” 

The New Economic Foundation report published today warned that expanding Luton and Gatwick would negate the benefits of the government’s climate plans made at Cop29 by 2050.

Expanding Heathrow would negate the Clean Power Project’s projected CO2 emission savings within five years of operation at expanded capacity, it added.

The think tank said that it is “highly unlikely that airport expansion, which primarily increases outbound leisure travel by frequent flyers, offers any growth benefits to the UK.”

A government spokesman said: “We are determined to get our economy moving and secure the long-term future of the UK’s aviation sector.”