Labour has threatened to withdraw support for the HS2 rail project if the bill rises above £50 billion.
Shadow transport secretary Maria Eagle said her party's backing depended on costs being kept under control.
The comments came amid growing speculation over the opposition's stance on the scheme.
The leadership insists it still believes HS2, which could see 225mph trains linking north and south by 2026, is vital.
But figures including Lord Mandelson and Alistair Darling have attacked the rising cost of the plans.
Last week shadow chancellor Ed Balls warned he would not write a "blank cheque" for the scheme.
Ms Eagle told the Sunday Times: "I am not willing to see this project start draining money from other vital rail projects - it's got to be delivered within the current budget.
"Nobody who is delivering it should be under any illusions that I will allow it to go up and up. That would put our commitment to it at risk.
"It shouldn't be going up above that £50 billion cap."
Ministers have estimated that the cost, including rolling stock, will be around £50 billion.
However, other research has suggested that the final bill for the project could reach £80 billion.
And Treasury officials are said to be privately working on a figure of £73 billion.
Transport Secretary Patrick McLoughlin insisted last week that David Cameron remained behind HS2, and expressed confidence there was still a cross party consensus in favour.Alison Munro, who is chief executive of HS2, said: "In numerous interviews in the last seven days I have stated categorically the costs of HS2 are firmly under control and that we fully intend to deliver Britain's new high speed rail network within the £42.6bn budget set by the Government.
"Passenger numbers continue to grow on our main north/south rail arteries and the time when we reach capacity crunch is approaching.
"There is no alternative that delivers the benefits of HS2. Through building a world class 21st century high speed rail network that will link our great cities as never before we will provide the capacity we need on those routes and free up space to expand commuter services and freight."