Controlling costs of HS2 will be "challenging", according to the Department for Transport (DfT), and has said cost estimates of the second stage of the project won't be set until 2019.
The comments were made in written evidence submitted by the DfT's permanent secretary Philip Rutnam, to support a Public Accounts Committee inquiry on the Great Western Railway.
Rutnam said it was an ambitious project and "like any programme of this scale, controlling costs will be challenging".
The 2015 Spending Review set a budget for HS2 of £55.7bn and Rutnam added that, despite the difficulties, the government "is committed to delivering HS2 within this budget".
In the evidence published on 11 January, Rutman also said "revised estimates" will be published in 2019 for the section of HS2 spanning Crewe to Manchester and West Midlands to Leeds, known as phase 2b.
The government announced its preferred route for that stage of HS2 in November, though the DfT is consulting on seven potentially substantial changes to the route, including a new connection to Sheffield's train station.
The consultations close in March, with the government set to publish its formal response later in the year.
It is expected that Phase Two will begin operating trains around 2033, with part of the route (between West Midlands and Crewe) to open in 2027.
Transport secretary Chris Grayling has said the full HS2 route will be "a game-changer for the country that will slash journey times and perhaps most importantly give rail passengers on the existing network thousands of extra seats every day".
Andrew Tyrie said the DfT needed to justify "why the latest projection for growth in demand will be any more reliable than its predecessor" and over the past seven years "we've had six of these - each different".
Whereas in November 2015, plans presented concluded a 2.2 benefit cost ratio, this has risen to as much as 2.7 in the latest government analysis.