Justine Greening and Vince Cable among those saying plan would jeopardise spending elsewhere

The true cost to the public of building a third runway at Heathrow has not been spelled out to taxpayers, according to a cross-party group of MPs, who warn that domestic flight connections and other transport spending will be jeopardised.

Justine Greening, who quit Theresa May’s cabinet in January, is among the MPs calling on the government to clarify what backing expansion at the London hub airport would mean, saying: “The transport secretary has a duty to spell out the true costs for taxpayers – and to be realistic about the benefits.”

In a letter to the Guardian, MPs and councils around Heathrow warn that promised domestic flight connections would only work with state subsidies that could not be guaranteed in perpetuity.

Third Heathrow runway would be bad for the whole UK Read more

Additionally, more than £10bn in additional rail and road spending to support a bigger airport would be laid on the public purse, they say. “What is certain is that taxpayers everywhere – including those living hundreds of miles away from the south-east – will all be paying for the expansion.”

Other signatories include Conservative and Labour MPs in west London constituencies, the Liberal Democrat leader, Vince Cable, and councils led by Richmond.

Having muted her opposition to Heathrow while in the cabinet, Greening, the MP for Putney and a former transport secretary, told the Guardian that Scottish support for the third runway was misplaced. “The SNP need to wake up to the threat that an expanded Heathrow poses to Scotland,” she said.

“A more expensive Heathrow means fewer connections. People in Scotland won’t understand why the Scottish government think that’s a good idea to support.”

Although Heathrow has pledged to keep charges “close to current levels”, and proposed a new scheme that reduces its own runway and infrastructure costs to £14.3bn, the government’s Airports Commission has forecast – and airlines fear – significant price rises to pay for it.

Greening said: “That’s what the modelling shows – and worse than that, the transport infrastructure to support expansion will be paid for by the taxpayer. That will mean that other investment projects that should have happened for the rest of the country will not get funding. The funding will be sucked in to another southern transport infrastructure project.”

Transport for London has estimated that the bill for additional rail and road infrastructure around an expanded Heathrow could be as much as £15bn.

Heathrow said the points raised by the MPs had repeatedly been shown to be unfounded. A spokesperson said: “Heathrow has already halved airport charges for domestic passengers and will set aside £10m in startup capital to fund new domestic routes. With expansion, we will create the capacity to connect all of Britain to global markets and it will be entirely privately funded at no cost to the taxpayer.”

A parliamentary vote is expected by summer 2018 on whether to approve the government’s national policy statement on aviation, which endorsed a third runway at Heathrow.