The Government is not opposed to Heathrow building a third runway if it meets four “clear tests”, Number 10 confirmed on Monday.
Chancellor Rachel Reeves has hinted that Labour would back the £14billion plan to boost economic growth.
But the Prime Minister’s spokesman said the proposals would have to meet air and noise pollution conditions before being approved.
“The Government’s number one mission is growing the economy and making people better off,” he said.
“The aviation sector clearly is a huge contributor to the economy. The Government's position is that it is not opposed in principle to new or longer runways or to people flying more.
“But there have to be clear conditions under which any future expansion would be justifiable and it would have to pass four clear tests.
“One of which is economic growth across the country. Secondly, it must ensure it is consistent with our binding climate change obligations. Thirdly, around noise pollution and fourthly around air pollution.”
There has been more than two decades of public wrangling over plans for a third runway at Heathrow.
The airport has faced fierce opposition from some MPs and Londoners, particularly those who live under flight paths, and was thought to have considered scrapping the project altogether after profits plummeted during the pandemic.
But earlier this year Heathrow bosses sped up their review of the third runway options after a much quicker than expected bounce back in passenger numbers.
Chief financial officer Javier Echave said it was looking at making a decision on restarting the programme - shelved during the Covid era - “as fast as we can” as the west London airport gets “fuller and fuller”.
The airport recently upgraded its forecast of the number of passengers expected to use the airport this year from 81.5 million to 82.4 million.
But the project is still likely to face opposition from some Labour MPs.
She warned it would present “a serious health issue” for her constituents, and called for the Government to block the project.
