Ministers must set out a realistic timetable for delivering the High Speed 2 rail project, MPs have warned.
The Public Accounts Committee (PAC) is "not convinced" that the target for completing phase 1 between London and the West Midlands by December 2026 will be met.
HS2 Ltd, the company responsible for building the £55.7 billion railway, has been asked by the Department for Transport (DfT) to asses the impact of delaying the opening until December 2027.
The report by the PAC warned that cost estimates for phase 2 - taking the line to Crewe (2a) and to Manchester and Leeds (2b) - are "still volatile" and exceed available funding by £7 billion.
It expressed doubts that proposed savings of £9 billion can be made "without adversely affecting the benefits of the programme".
The DfT responded to the report by insisting that the rial project is "on time and on budget".
The PAC described HS2 Ltd's recent recommendation to the DfT that the planned station in South Yorkshire should be moved from Meadowhall to Sheffield Midland station as an example of the "significant uncertainty that remains about plans for phase 2".
But it also acknowledged that "considerable progress" has been made with preparations for the project since 2013.
Labour MP and chair of the committee, Meg Hillier, said: "The Government has promised significant benefits to taxpayers in return for their investment in HS2, expected to run to more than £55 billion.
"Despite this, Parliament and the public are still in the dark about crucial details - not least when the railway will open, how much it is expected to cost and precisely where it will go."
She claimed the "uncertainty enveloping a project on which strong and stable leadership is vital" was exacerbated by the announcement over the weekend that HS2 Ltd chief executive Simon Kirby was leaving the company.
Ms Hillier went on: "Government must explain its intentions and the basis for its decisions in a transparent manner.
"The public must be confident the grand vision for HS2 does not blind the Government to the finer points which have implications for many people's lives now and in the decades to come.
"Local authorities must know central Government's intentions to ensure they can plan effectively for regeneration and maximise the potential for growth near HS2 stations.
"The Government is due to announce its decision on the 2b route this autumn and we urge it to seize this opportunity to address the concerns set out in our report."
A DfT spokesperson said: "The Government is fully committed to HS2 and the project is on time and on budget.
"We are keeping a tough grip on costs, and pressing ahead with plans for phase 2, with further details due to be announced this autumn.
"Improving regional infrastructure is vital in supporting regional growth and building an economy that works for everyone. HS2 is a key part of this, and will be the backbone of our national rail network."
HS2 Ltd issued a statement which read: "We are making good progress in building confidence in our programme plan, and will continue to do so over the coming months.
"We expect to get a Government decision on the 2b route this autumn and we are on track to achieve Royal Assent by the end of the year."