Ed Balls today signalled a retreat on Labour's support for High Speed 2.
The shadow chancellor used his conference speech in Brighton to question whether the rail project should still go ahead.
"The question is not just whether a new high speed line is a good idea or a bad idea, but whether it is the best way to spend £50 billion for the future of our country," he said.
"In tough times when there is less money around and a big deficit to get down, there will be no blank cheque from me as a Labour chancellor for this project."
He suggested that the government's continued support for the project was due to "vanity".
"David Cameron and George Osborne have made clear they will go full steam ahead with this project – no matter how much the costs spiral up and up," he added.
"They seem willing to put their own pride and vanity above best value for money for the taxpayer."
Labour's shadow business secretary Chuka Umunna also questioned the project's viability today.
"We will not just sign a blank cheque on it," he told a fringe event in Brighton.
"And if it gets to the point on cost where [you ask] is there a better way to spend the money… there is not an infinite amount of money on this."
This is not the first time that Labour has questioned the funding for HS2. However, Balls and Umunna's comments are the clearest indication so far that the party is considering removing their support for the project.
It follows a series of other interventions from senior Labour figures, including former chancellor Alistair Darling, questioning the future of HS2.
Darling said last month that HS2 would drain investment from other projects, causing a "nightmare" on the rail network.
It also follows a recent damning report from the public accounts committee.
The committee's Labour chair Margaret Hodge said earlier this month that the government "has not yet demonstrated that this is the best way to spend £50 billion on rail investment in these constrained times".
Other figures in the party remain committed to the project, however.
Lord Adonis, who first announced Labour's commitment to the project in 2010, has warned that it would be "an act of national self-mutilation" for Labour to ditch their support.