MPs say they are "sceptical" about whether the government's HS2 rail line will deliver value for money.
The Commons Public Accounts Committee said the £50bn price tag included a "generous contingency" which could be used to mask cost increases.
Ministers lack a "clear strategic plan for the rail network", they added.
The Department for Transport said HS2, which will link London with the north via Birmingham, would have a "transformational effect".
A spokesman for the department said the government was "fully focused on keeping costs down".
The first phase of HS2 will be between London and Birmingham opening in 2026, followed by a V-shaped section to Manchester and Yorkshire.
The MPs said they were concerned about the government's ability to deliver on time and on budget.
The contingency gives the government a 95% assurance of delivering within budget, the committee said, but it added: "We are concerned that this will simply be used to mask cost overspends, rather than valid calls on contingency funds."
This week HS2 Ltd executive chairman Sir David Higgins told a House of Lords committee: "We should not assume the contingency will be spent."
The committee also questioned the recent government backing for an "HS3" link between northern cities.
Labour MP Margaret Hodge, who chairs the committee, said the government had not assessed HS3 before giving the go-ahead to HS2.
The report said this suggested the Department for Transport (DfT) took "a piecemeal approach to its rail investment, rather than considering what would benefit the system as a whole and prioritising its investment accordingly".
Ms Hodge said it was "hugely important" to make properly-informed judgements.
"Yet the government takes decisions without a clear strategic plan," she added.
The report also said the government had yet to publish proposals for how Scotland would benefit from HS2.
A DfT spokesman welcomed the committee's scrutiny.
He said: "HS2 will have a transformational effect, rebalancing the economy and helping secure the UK's future prosperity, providing high value for money to the taxpayer."
The DfT stressed its determination to build the line on time and within budget.