The estimated cost of building HS2 between London and Birmingham has soared to as much as £66.6 billion, MPs heard.
HS2's executive chairman Sir Jon Thompson told the Transport Select Committee the estimated cost for phase one was between £49 billion and £56.6 billion at 2019 prices but adjusting the range for current prices involved "adding somewhere between eight and 10 billion pounds".
In 2013, HS2 was estimated to cost £37.5 billion (in 2009 prices) for the entire planned network, including now-scrapped extensions from Birmingham to Manchester and Leeds.
Sir Jon said reasons for the cost increase included original budgets being too low, changes to scope, poor delivery and inflation.
He said: "This is a systemic problem. It's not just about HS2, it's about large projects that the Government funds.
"The budget needs to be set early on in order for an outline business case to be approved by the Government, sometimes by Parliament. At that point, people think the original estimate for phase one was £30 billion-something.
"That is based on very, very immature data. You don't have a design, you haven't procured anything, there is no detail on which you can cost anything."
He added: "If you say to a builder, can you give me a quote for an extension, they walk around and say 'it's £50,000-something'.
"But then you get into the detailed design, you know exactly how big it is, what surfaces you want, how much concrete needs to be poured. Unsurprisingly, you get a better number.
"That's the situation here. The situation with HS2 in my opinion is the estimate was poor, the budget was set too early and then, when you get further into it, you get much better information. Then, on that basis, you can cost it out with more accuracy and then you discover it's higher."
On the issue of why official cost estimates are still being given at 2019 prices, Sir Jon said: "It is the Government's long-standing policy that infrastructure estimates are only updated at Spending Review points.
"That's why we're still working to 2019 prices and the whole conversation about 2019 which is an administrative burden of some significance in the organisation.
"All of the invoices we get we have to then deflate backwards to 2019 prices even though we're paying them at 2024 prices and then we have to adjust the accounts to account for that so it is a significant administrative faff."
Joanna Marchong, investigations campaign manager of the TaxPayers' Alliance, said: "The continued soaring costs of HS2 are a slap in the face for taxpayers.
"Even with the termination of extensions, this project continues to look like a financial trainwreck. If HS2 is to go ahead at all, ministers must finally get spending on track."
