Revised plans for HS2 should not be put into action until the government is confident they can be delivered, according to the public spending watchdog.

The project to build the high-speed railway must be put on a stable footing to avoid a repeat of past failures, the National Audit Office (NAO) said in a report.

Last month, the transport secretary, Heidi Alexander, announced that the scheme would now cost up to £102.7bn and trains would not start running between London and Birmingham until as late as 2039 – £70bn more and 13 years later than originally promised. She said the project would not be entirely completed until as late as 2043.

After a 15-month review, Mark Wild, the chief executive of HS2 Ltd, came up with a new detailed plan for the remaining work.

The NAO said a “considered approach” was being taken to resetting HS2, but added “significant work” remained before the project was completed.

The report said the Department for Transport (DfT) and HS2 Ltd were aiming to complete the reset by spring 2027, but added: “It is crucial that they get it right this time following past failures.

“They should ensure that they do not proceed with putting the plans into action until they are confident everything is in place to deliver against them.”

The NAO said the DfT and HS2 Ltd should review in autumn how “realistic” the new timetable was and revise it if necessary. It estimated the cost of the reset process would be £153m.

Constructing HS2 from London to Birmingham, as well as the now abandoned onward legs to Leeds and Manchester, was initially estimated to cost £32.7bn at 2011 prices. The latest cost estimate is roughly double the figure estimated in 2020. Services were initially scheduled to begin this year.

The Manchester HS2 leg was cancelled in October 2023 by the then prime minister, Rishi Sunak.

The NAO said most of the cost increases were caused by “cost underestimation, inefficient delivery and scope changes”.

Gareth Davies, the head of the NAO, said: “Establishing a fully robust estimate of cost and schedule, completing commercial negotiations and getting the right capabilities in place is necessary before they [DfT and HS2 Ltd] can complete the reset.”

A DfT spokesperson said: “Following years of mismanagement, this government has taken decisive action to reset HS2 and ensure the safe delivery of the line between Birmingham and London at the lowest reasonable cost.

“The reset is driving faster, more efficient construction on the ground, with six major construction milestones reached ahead of schedule last year.”

A spokesperson for HS2 Ltd said: “Fundamentally resetting HS2 was the only way to regain control of the project and break the cycle of poor delivery, delays and cost increases.

“This is a hugely complex task, requiring a vast amount of external industry expertise, and has been carried out in parallel with an increase in productivity across HS2’s vast 140-mile construction programme.

“Any costs associated with the reset will ultimately pay for themselves through improved management and efficiencies.”