Heathrow "played an absolute blinder", MPs heard, after it emerged that the government would pay costs if expansion did not go ahead.

The "poison pill clause" is part of an agreement signed by the airport and Department for Transport that would allow Heathrow to recover its costs if an alternative scheme had been preferred by the government or if the government withdraws support to expand Heathrow.

In the event the third runway at Heathrow is not built, "taxpayers could be left picking up a bill of multiple millions of pounds" according to the shadow chancellor.

The clause in question was highlighted by Putney MP Justine Greening who questioned Transport Secretary Chris Grayling over the phrasing on Tuesday, when the government's backing of Heathrow's bid was announced.

She then repeated the question over the "bail-out" at Prime Minister's Questions on Wednesday (June 6), the day after the government confirmed its support for expansion at Heathrow , ahead of a vote of MP's.

Justine Greening questioned the "poison pill clause" which could see the government paying the airport's costs in Heathrow doesn't get permission from parliament

The Tory MP told Theresa May that the airport had "played an absolute blinder" in securing the agreement in the Statement of Principles in 2016.

"It has somehow managed to get a poison pill clause agreed by the DfT that means the taxpayer has to cover all its costs if things go wrong" she said.

"Is this not the worst kind of nationalisation - the public sector and taxpayers owning all the Heathrow downsides and risks, and the private sector owning all the upside and the financial returns?".

Following the question, Hayes and Harlington MP and Labour Shadow Chancellor John McDonnell wrote to the Transport Secretary to ask about the cost recovery clause, which has no end date.

"I am extremely concerned that through this agreement the government has tied itself into a considerable liability that could fall upon taxpayers shoulders and pre-empts the decisions of Parliament on this matter," he wrote in his letter.

The National Airports Policy Statement is due to be published in the coming weeks, after which Parliament has 21 days to vote on the findings, including selecting Heathrow as the official option for airport expansion.

Transport Secretary Chris Grayling told parliament that the cabinet had approved the third runway proposal

Legal action is likely following Cabinet approval, with many residents' and environmental groups opposed to the runway over concerns about noise and emissions.

They are expected to launch a High Court challenge on the grounds that the Government's decision to support the scheme is unlawful.

Paul McGuinness, Chair of the No 3 Runway Coalition, said: Perhaps it’s little wonder that Heathrow has been spending money on expansion, as if it had nothing to lose, long before the government adopted it as policy.

"But while it’s broadly accepted that the whole process of selecting a site for extra capacity was only ever about Heathrow, this is a concession too far.

"We know there are doubts about Heathrow’s ability to fund the project, but any bail out agreement for Heathrow, however vague, is potentially a massive burden for taxpayers and it needs to be disclosed”.

A DfT spokesman said: “The government has made clear that it believes a new northwest runway at Heathrow is the best scheme to deliver the economic and connectivity benefits this country needs.

“It will be privately financed and the costs will not fall on the taxpayer. The point around potential financial liability has been taken out of context from a non-legally binding document, which makes clear that it gives Heathrow no legal right to any costs or losses from the government should their scheme not proceed.”