David Cameron faced the prospect of being left isolated on the High Speed 2 (HS2) project, after an intervention by Peter Mandelson suggested Labour could be about to turn against the idea.
The former business secretary warned the rail project could be a "monumental mistake" which shifted resources from the north to the south-east.
"I now fear HS2 could be an expensive mistake," he wrote in the Financial Times.
"It perversely represents a shifting of rail resources away from the north to the south-east commuter belt.
"All the parties – especially Labour – should think twice before binding themselves irrevocably to HS2. It is not all it seems and has the potential to end up a mistake, damaging in particular to those people that it was intended to help."
The former European commissioner said Labour's decision to back the project was driven in part by a desire to appear forward-looking in the wake of the financial crisis, but that it did so on the basis of "speculative" costs.
"Perhaps the most glaring gap in the analysis presented to us at the time were the alternative ways of spending £30 billion," he added.
Labour had also assumed much of the funding would come from the City, rather than entirely from the taxpayer.
Mandelson's concern about costs echoes that of several business people and MPs. Transport secretary Patrick McLoughlin admitted last week the price tag had risen from £34.2 billion to £42.6 billion.
Labour sources took to Twitter last night to stress that the party's position was not changing, but Mandelson's intervention comes amid reports that Ed Balls is becoming wary of the project, as is Maria Eagle, who holds the shadow brief on the project.
Liberal Democrat transport minister Norman Baker said: "Thankfully, since Blair and Brown are gone the country no longer has to do whatever Peter Mandelson says. And given the fiasco of the Millennium Dome, for which he had personal responsibility, I hardly think he is the best person to offer advice on large projects anyway.
"More and more people are using the rail network every year so we desperately need more north-south capacity – unlike Peter Mandelson we can’t all hop on a private jet."
HS2 is a potentially explosive political issue for the Conservatives, because its opponents mostly come from the 'shires', where the party has its core support.
Ukip has made significant gains by opposing the project as a way of winning over former Tory voters.
Cameron has relied on the cross-party consensus in favour of the project to escape many of the criticisms, saying it is a vital component to the UK's ability to succeed in the 'global race'.