Ed Balls has dropped the heaviest hint yet he would like to scrap the High Speed 2 rail project were Labour to win the next election, opening up a split at the top of the party.
The shadow chancellor told the Labour Party conference in Brighton on Monday afternoon that the HS2 line had been "totally mismanaged" by the coalition - leading its costs to reach £50bn.
"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. They seem willing to put their own pride and vanity above best value for money for the taxpayer," he said.
"Labour will not take this irresponsible approach. So let me be clear, 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 or for any project.
He added:"Because 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."
Balls' words are the harshest criticism yet for the project, which began life under the last Labour government.
His attack on the rail line, which the government argues is necessary to bridge the North/South divide as well as free up much needed rail capacity, appears to be at odds with recent statements by other senior Labour figures.
Just last week shadow transport secretary Maria Eagle told The Huffington Post UK that there was "no way other than building a new line" to deal with the nation's transport needs.
"None of those people who either object to specific routes or object to spending all of this money on building extra capacity into our railway system the can explain how they are going to deal with this capacity crunch," she said.
She added: "Ed Miliband, Ed Balls and I agree it needs to go ahead."
And while Eagle acknowledged there was "of course" a point where the project would cost to much to be worthwhile - she said there was "no reason" the line could not be delivered within the current £50bn budget. The figure Balls today questioned.
Lord Adonis, the Labour peer who was transport secretary when the rail project was conceived, said in August it would be an "act of national self-mutilation to cancel HS2". Writing in the New Statesman he said: "The case for High Speed Two is as strong now as when Labour committed itself to the project in March 2010."