The timetable for planning phase one of the HS2 project - from London to Birmingham with work due to start in 2016/17 - was "challenging", the National Audit Office report said.
This challenging timetable "makes delivering this work difficult and increases the risk that the programme will have a weak foundation for securing and demonstrating success in the future", the report said.
Expressing "reservations" about the business case for HS2, the NAO said the Department for Transport's (DfT) methodology for appraising the project put a high emphasis on journey-time savings, from faster and more reliable journeys.
But the report added that the relationship between these savings and the strategic reasons for doing the project, such as rebalancing regional economies, was unclear.
The NAO said it was also unclear whether the business case covered just phase one or the full route including phase two - the Y-shaped network from Birmingham to Manchester and Leeds due to open in 2032/33.
More top news
Philip Thomas doesn't like to dwell on the disaster that buried him alive - but he can still recall in detail how an unpaid debt saved him.
The deal would set up a free trade zone between the EU and Canada - and could be an important test for the UK's Brexit negotiations.
A mother has won her battle against the Home Office to allow her Nigerian sister into the country - so she can undergo a marrow transplant.