Theresa May’s plans for Heathrow expansion are facing a knife-edge Commons vote after Labour signalled the project currently failed key tests on jobs and the environment.

Jeremy Corbyn’s spokesman told HuffPost UK that “so far” the Government’s proposals for a third runway had not met the party’s four conditions on noise levels, climate change, aviation capacity and improved connections to the regions of the UK.

The tough message confirms for the first time that Labour may refuse to vote for the plans when they come before MPs for approval in the next four weeks.

With Boris Johnson expected to be out of the country and several Tory MPs poised to rebel, ministers were hoping to rely on Labour to avoid a possible defeat on the much delayed aviation plan.

However, Corbyn’s spokesman said the issues of air pollution, noise for residents, regional connections and greater capacity were crucial. 

Asked by HuffPost if the test had yet been met, he replied: “We’re in favour of airport expansion in the south-east, but it has to meet these four tests.

“So far, from what we’ve seen, it looks as though the existing proposals don’t do that.”

His words suggest Labour could either whip a vote against the plans or at least order its MPs not to back the Government. 

That could leave the PM relying on the SNP, which has so far refused to say which way it will vote.

Shadow Chancellor John McDonnell is the leading opponent of Heathrow within Corbyn’s top team, though many Labour MPs have gone on record to support its expansion.

Transport Secretary Chris Grayling gave the go ahead for the expansion this week, ending decades of postponements.

In a further sign of backbench opposition, former Cabinet Minister Justine Greening warned Theresa May on Wednesday that taxpayers were in danger of picking up the multi-billion pound bill for the private airport operator if the Heathrow scheme collapsed.

Greening, who has long opposed a third runway, used Prime Minister’s Question time to warn May that there was a nasty surprise buried in the aviation proposals, with one clause leaving taxpayers liable if the scheme falls apart.

“Heathrow have played an absolute blinder with the DfT [Department for Transport]. ..they’ve somehow managed to get a poison pill clause agreed by the DfT that means the taxpayer has to cover all their costs if things go wrong.

“Isn’t this the worst kind of nationalisation – the public sector and taxpayers owning all of the Heathrow downsides and risks but the private sector owning all of the upside and financial returns?”

May insisted the plan “demonstrates this government’s commitment to delivering the jobs and major infrastructure that this country needs to thrive”.

The PM stressed that the expansion of the airport would be “fully financed by the private sector” and appeared to reject Greening’s claims.

“The Statement of Principles [signed between Heathrow and the Government] is clear that it does not give Heathrow Airport Ltd the right to claim any costs or losses from government should their scheme not proceed,” she said.

Greening was so furious at that reply that she later raised it in a Parliamentary point of order, accusing DfT officials of having “misinformed the Prime Minister” about the deal.

She cited a clause in department’s 2016 agreement with Heathrow which says the airport’s owners can “pursue any and all legal and equitable remedies” in the event that ministers back an alternative scheme or withdrawal their backing for expansion.