John McDonnell has joined Conservative rebels against Heathrow’s third runway to attempt to persuade MPs who may still have reservations about the project to vote against it.

The shadow chancellor, a longstanding opponent of expansion, is working with the Tory former transport secretary Justine Greening and the backbench MP Zac Goldsmith, both also firm critics, to try to halt the government’s expansion plan.

At an emergency meeting in the Commons on Tuesday after the transport secretary, Chris Grayling, confirmed that the government would back a third runway, the cross-party group agreed their strategy ahead of the key vote, which is expected to take place in about two weeks’ time.

McDonnell has highlighted the potential multimillion-pound cost to the taxpayer in the event that the government, including any future Labour administration, should change its mind on expansion.

In a letter to Grayling, he raised concerns over how much, if any, of the £14bn project would be publicly subsidised if Heathrow’s plans for a third runway did not proceed.

McDonnell wrote: “There appears to be no end date to this agreement. This means that taxpayers could be left picking up a bill of multiple millions of pounds if the government does not proceed with developing the Heathrow north-west runway scheme.

“I am extremely concerned that through this agreement the government has tied itself into a considerable liability that could fall upon the taxpayers’ shoulders and preempts the decision of parliament on this matter.”

The senior Labour MP, whose Hayes and Harlington constituency includes Heathrow, asked the Department for Transport to reveal whether it had made any assessment of the financial liabilities that taxpayers could be exposed to.

The DfT signed a cost recovery clause with Heathrow in 2016 in its statement of principles, which said the airport would be able to pursue “legal and equitable remedies (including cost recovery)” if the government opted for an alternative scheme or withdrew its support.

The issue was raised by Greening in the Commons earlier this week, when she warned that taxpayers could end up with a bill costing “billions and billions” of pounds. Grayling did not respond.

Labour has said it would consider whether its four tests on airport expansion had been met before deciding whether to back the plans, raising the prospect of the government being defeated on Heathrow expansion in the Commons.

McDonnell tweeted on Tuesday: “I remain implacably opposed to expansion at Heathrow and after listening to the transport minister, Chris Grayling, today I am

even more convinced that this would be a costly, environmental and social disaster that will never be built.”

A Labour source said: “John doesn’t believe that the tests have been met yet and is quite confident that is not going to change.”

However, many backbench Labour MPs and the unions support a third runway. Len McCluskey, the general secretary of Unite, the party’s biggest union backer, was quoted in Heathrow’s official response to the announcement.

Jeremy Corbyn’s spokesman confirmed that the Labour leadership believes the government’s plan for a third runway does not pass the party’s “four tests”, increasing the likelihood that Labour will vote against it, and leaving Theresa May reliant on SNP and Labour votes to get it through.

“We’re in favour of airport expansion in the south-east but it has to meet those four tests and so far what we’ve seen, it looks like the existing proposals don’t do that, but we obviously need to see the detail,” he said.

He said that “several” of the tests, which cover capacity, noise and environmental impact, climate change and regional connectivity, had not yet been met, adding that the government doing so was a “clear requirement” of successful airport expansion.

A DfT spokesperson 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.”

Pippa Crerar, Guardian, 6 Jun 2018