The NAO is also far from convinced about the Department for Transport ‘s business case for the scheme.
The current estimated cost of phase one is between £15.4bn and £17.3bn.
The NAO estimates that there is a £3.3 billion funding gap over four years from 2017 to 2021 which “the government has yet to decide how to fill.”
Amyas Morse, head of the National Audit Office, said: “It’s too early in the High Speed 2 programme to conclude on the likelihood of its achieving value for money.
“Our concern at this point is the lack of clarity around the Department’s objectives. The strategic case for the network should be better developed at this stage of the programme.
“It is intended to demonstrate the need for the line but so far presents limited evidence on forecast passenger demand and expected capacity shortages on existing lines.
“It is also unclear how High Speed 2 will transform regional economies by delivering jobs and growth.
“The Department is trying against a challenging timetable to strengthen its evidence and analysis, which at present provide a weak foundation for securing and demonstrating success in the programme in future.”