The £56bn HS2 high-speed rail project is running over budget and the opening date for the first part of the line is "at risk", according to the National Audit Office (NAO).
The government accounts watchdog labelled plans to open the first section of the line, between London and Birmingham, by 2026 as "unrealistic".
The NAO said that phase one of the controversial project is already running £204m (€245m, $272m) over its £27.4bn budget.
The second phase of the scheme — between Birmingham, Manchester and Leeds — is £7bn over budget and is set to be completed by 2033. However, the Department for Transport said £2bn of savings have so far been made in this part of the project.
The NAO said: "The 2026 target opening date for phase 1 is at risk despite good progress with some major procurements."
The project involves issuing a series a major civil engineering contracts. But the firm tasked with completing the complex project, HS2 Ltd, said its plans were on track despite "many challenges".
HS2 Ltd chief executive Simon Kirby said: "The role of the NAO is to challenge projects such as HS2 and through that challenge improve the way they deliver for the taxpayer. This report does this and we accept that challenge."
Minister of State for Transport Robert Goodwill said: "HS2 will transform our country, generating jobs and growth and helping to rebalance our economy."
NAO head Amyas Morse said added the Department of Transport "now needs to get the project working to a timescale that is achievable". HS2 faces opposition from a number of local groups and a series of previous studies have cast doubt over the project's value for money.
The NAO report was written before this week's Brexit vote, so it doesn't speculate on what effect, if any, it could have on HS2.