The HS2 high-speed rail project has an "unrealistic timetable" and faces major cost pressures, a report by the National Audit Office has said.
The £56bn programme has too ambitious a schedule and rising costs could mean that not all its intended benefits are delivered, the NAO said.
The NAO said the 2026 target opening date for the first phase was "at risk, despite good progress".
But HS2 Ltd said it was confident that it could achieve its objectives.
The first phase of HS2, linking London and the West Midlands, is due to be completed in 10 years' time.
The next section, running as far as Crewe, is due to open a year later in 2027, with a further extension to Manchester and Leeds due to be finished by 2033.
The project cleared its final Commons stage in March and is now being considered by the House of Lords.
Analysis: Richard Westcott, BBC transport correspondent
The NAO report was written before the Brexit vote, so it doesn't speculate on what effect, if any, it could have on HS2.
Still, this behemoth of a project involving £56bn of public money would appear to be safe, for now.
The Department for Transport's very keen to send out a "business as usual" message, promising that planned investment in our roads and railways won't suffer.
MPs are due to vote on phase one of HS2 later this year. If they approve the bill, that's effectively the green light to start building the line from London to Birmingham. And MPs approved the project overwhelmingly the last time they had a vote.
The company building the line, HS2 Ltd, is also about to hand out £11bn worth of contracts in the next couple of months. The winning companies can't begin building work until the HS2 bill has royal assent, due by the end of the year, but they will be looking to recruit, hire vehicles, get plans in place.
Having said all of that, who'd be brave enough to predict anything right now.
"HS2 is a large, complex and ambitious programme which is facing cost and time pressures," said Amyas Morse, head of the NAO.
"The unrealistic timetable set for HS2 Ltd by the Department [for Transport] means they are not as ready to deliver as they hoped to be at this point.
"The Department now needs to get the project working to a timescale that is achievable."
The NAO said phase one of HS2 was currently forecast to cost £27.4bn, exceeding available funding by £204m.
The estimated cost of phase two was also over budget, the NAO said, although potential savings had been identified.
The NAO added that integrating HS2 into the wider UK rail network "is challenging and poses risks to value for money".
In its response, HS2 Ltd it welcomed the report "and its confirmation that, whilst many challenges remain, the project is on track to deliver its strategic scope and to do so on budget".
"The role of the NAO is to challenge projects such as HS2 and through that challenge, improve the way they deliver for the taxpayer," said the chief executive of HS2, Simon Kirby.
"This report does this and we accept that challenge. It also, however, recognises the real progress we have made in taking the concept of HS2 and moving it nearer reality."
David Higgins, the non-executive chair of HS2 Ltd, told BBC Radio 4's Today programme that the timetable was "tight", but added: "We know the project can be delivered in that time."
He said the projected overspend on phase one represented less than 1% of the cost, which was not excessive "on a project of this scale at this stage".
Transport Minister Robert Goodwill said: "HS2 is on track and the National Audit Office agrees. We have strong cross party support and are on schedule to gain the powers needed to start building HS2, which the NAO acknowledges is a significant achievement."