The High Speed 2 rail project is under financial strain and could be delayed by a year, the Whitehall spending watchdog said.
The National Audit Office (NAO) warned that the 2026 target date for opening phase one between London and the West Midlands is "at risk".
It revealed that the Department for Transport (DfT) has asked the company responsible for building the railway, HS2 Ltd, to assess the impact of extending its timetable by up to 12 months.
The DfT has also called on the firm to revisit its schedule in a bid to increase its confidence of completing phase one on time from 60% to 80% without increasing costs.
But Transport Minister Robert Goodwill insisted that HS2 is "on track" despite the NAO warning that the £55.7 billion railway is facing financial pressures.
Cost forecasts for phase one exceed available funding by £204 million, while phase two - taking the line to north-east and north-west England - contains some elements that are "currently unfunded", the report said.
It went on to warn that the benefit-cost ratio of the scheme could fall from 1.7 to 1.5 if the programme is not delivered well within available funding.
The NAO noted that although significant progress has been made by the DfT and HS2 Ltd in preparing to deliver the project - including some major procurements for phase one and processing 2,600 petitions - the timetable set by the Department was "too ambitious".
The head of the NAO Sir Amyas Morse said: "HS2 is a large, complex and ambitious programme which is facing cost and time pressures.
"The unrealistic timetable set for HS2 Ltd by the Department 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."
Meg Hillier, chair of the Commons Public Accounts Committee, commented: "Today's NAO report shows that preparations to deliver High Speed 2 are under severe strain.
"The Department for Transport has, yet again, set unrealistic timetables for delivering this programme and HS2 Ltd is only 60% confident that it will complete phase 1 by December 2026."
She added: "The programme is at a critical juncture and the Department has some major decisions to make if it is going to protect taxpayers' interests and deliver all that the Government has promised from HS2."
But Mr 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.
"We are on course to start construction next year and complete the scheme on time and budget."