Allies of Labour leader Jeremy Corbyn suggest he will oppose Heathrow expansion.

Corbyn and his shadow chancellor have long opposed a third runway.

Labour has set four tests that expansion must meet for their support.

Source close to Corbyn denies to Business Insider that a decision has been made.

LONDON — The future of Heathrow is in doubt following reports that Labour are preparing to vote against the expansion of the airport when it comes before Parliament early next year.

While not committing to Heathrow expansion, Labour's general election manifesto did commit to expanding airport capacity in the South East and welcomed work done by the Airports Commission, which backed expansion at Heathrow.

However, allies of the Labour leader Jeremy Corbyn have told the Financial Times that the party is now unlikely to back a third runway at Heathrow.

"It has to pass our tests and no one here expects that to be likely,” one source told the paper, with another adding that "If the vote is any time soon there is no way we would back it, mainly on the basis of air quality, and that’s unlikely to change any time soon."

Both Corbyn and his shadow chancellor John McDonnell have long been opposed to expansion, as is the current London mayor Sadiq Khan, while most Labour MPs are in favour.

John McDonnell has long opposed Heathrow expansion. Chris Ratcliffe / Getty A source close to Corbyn denied reports that Labour would oppose expansion and said that no decision had yet been made by the leadership.

"We will assess any proposals against our four tests and have not prejudged," they told Business Insider.

Under Labour's manifesto, new airport capacity will only be backed if it meets the following four tests:

Noise issues are addressed

Air quality is protected

The UK’s climate change obligations met

Growth across the country is supported.

It was reported last year that Labour plans to give its MPs a free vote on Heathrow, which could allow expansion to pass. However, a spokesperson for Corbyn today refused to comment on whether the party still planned to allow MPs a free vote.

"We don't comment on the whip," they said.