Treasury minister Danny Alexander has said he is "very confident" the HS2 high-speed rail project will be delivered within its £42.6bn budget.
He told the BBC's Andrew Marr Show the government was using the "same techniques" to ensure efficiency as for the 2012 London Olympics.
Supporters say the plan to link London to Manchester and Leeds will reduce journey times and boost growth.
But Labour's deputy leader Harriet Harman said costs had to be monitored.
The aim is to get trains running as fast as 250mph (400km/h) between London and Birmingham from 2026, with branches to Manchester and Leeds via Sheffield planned for 2033.
The estimated cost of the project has risen from £34.2bn to £42.6bn - plus £7.5bn for rolling stock.
Mr Alexander, Chief Secretary to the Treasury, insisted this would not change, saying: "The real cost is the budget that we set out in June this year: £42.6bn. It hasn't changed at all.
"That number includes within it a significant amount of contingency.
"I'm very confident that, as we work through the project and deliver it, we will not just deliver it within that budget but, like the Olympic Stadium project, under budget too.
"That is something I'm working very, very hard to make sure happens. We are applying the same techniques we used to deliver the Olympic Park to the HS2 project."
Mr Alexander added: "We have set that budget and we will stick to it."
For Labour, shadow chancellor Ed Balls is reported to have compared HS2 to the Millennium Dome, saying: "I think you should learn from your mistakes."
But Ms Harman played down the comments, saying Mr Balls had been asked about the Dome by a Mail on Sunday journalist and had not volunteered the comparison.
She told the Andrew Marr Show: "We absolutely support better north-south lines. We are in favour of rail infrastructure for commuters and also for long-distance travellers and freight but not at any cost.
"And what Ed Balls is saying is we have to keep a strong eye on the costs as well as on the benefits.
"It's no good the government simply complaining about people who are raising these issues. They should be addressing these issues, controlling costs and being properly analytical about the benefits that are available."
Shadow energy secretary Caroline Flint told BBC One's Sunday Politics: "HS2 has never had a blank cheque from the Labour Party... We have to look for value for money and we have to look at how it benefits the country."
Bob Crow, general secretary of transport union RMT, said the "political posturing" over HS2 was a "smokescreen" designed to delay investment in the railways.