There is “no clear end in sight” to HS2 costs and delays, MPs on the Public Accounts Committee have said.
The committee is “increasingly alarmed” about key parts of the project, including a lack of progress at Euston Station.
Without a government decision on the station, the project “will literally run out of time”, the committee fears.
The Department for Transport (DfT) said it was making significant progress delivering HS2.
Euston Station is an important part of the first phase of the rail project, both as a London terminus and as a link to other infrastructure such as the London Underground.
But the DfT “is yet to make key decisions on the design and approach to construction there” despite having the necessary planning consents.
This could “lead to yet more costs, delays and uncertainty over the promised benefits of the programme”, the committee said.
Dame Meg Hillier, chair of the Public Accounts Committee, said: “HS2 is already one of the single most expensive taxpayer-funded programmes in the UK but there’s actually no clear end in sight in terms of the final cost, or even the final route.
“The project was plagued by a lack of planning and transparency from the start and there are many difficulties ahead.
“This project cannot simply keep sinking more taxpayer funds without greater clarity on the later phases. The development of Euston is a real challenge that must be resolved swiftly now.”