An extended northern runway at Heathrow could cut CO2 emissions from aircraft operations by 37%, backers of the scheme claim.

The reduction of as much as 210,000 tonnes a year would come from reduced delays and congestion when the airport is running at full capacity.

Heathrow Hub believes extending one of the airport’s two parallel runways rather than building a new third strip would assist Heathrow and airlines in meeting legal obligations to reduce emissions and position the UK as a leader in the move to net zero emissions.

Extending the northern runway, at a cost of £4.3 billion for its first phase, is much cheaper than the projected £38 billion third runway, meaning it would not be reliant on additional air traffic movements for funding.

The additional capacity would make Heathrow more efficient and resilient from its existing operations and any new capacity need not be released unless environmental targets are met, according to Heathrow Hub.

The Court of Appeal has already ruled that the Airports National Policy Statement (ANPS) was unlawful on environmental grounds. The result of an appeal by Heathrow to the Supreme Court over the decision to block the build of a third runway is expected in January.

Jock Lowe, who heads the Heathrow Hub proposal, said: “Reducing carbon emissions is now a legal duty for Heathrow and the airlines. Our proposal would help them deliver it.

“It has become increasingly clear that Heathrow’s third runway will not go ahead. Whatever the result of the Supreme Court case, it is too expensive, too complicated and totally incompatible with the government’s net zero emissions by 2050 target.

“By contrast, our extended northern runway proposal, can help make Heathrow more efficient and resilient, reducing congestion and emissions and saving time and money. Its low cost and incremental approach spreads risk and ensures new flights need only be allowed once environmental and noise targets have been met and demand resumes.

“Unlike the third runway, the extended runway is a detailed, fully-costed scheme with a safety-case. It would also incentivise additional financing from Heathrow’s shareholders.

“Our proposal would create a more sustainable global hub, well positioned and ready for future growth when the time comes.”

A spokesperson for the airport said at the time the appeal to the Supreme Court was lodged in October: “Heathrow will ensure the expansion project is compliant with the UK’s climate change obligations, including under the Paris Climate Agreement, as part of our plans to reach net-zero carbon.

“We fully expect to be held to account by government through the planning process.

“The ANPS makes clear that approval for expansion would be refused if it would have a material impact on the UK’s ability to meet its carbon reduction obligations.

“We’re appealing to the Supreme Court to allow this thorough planning process to proceed as it was designed.

“Given the timescales required to deliver complex infrastructure of this scale in the UK, it’s critical that we get on with laying the groundwork today for future operations that will be essential for a successful global Britain in the decade after Brexit.”