The timetable set for the government to deliver the controversial HS2 rail project is "unrealistic", the National Audit Office has said.
The government department said the £56 billion programme was too ambitious with many cost and time pressures.
The current HS2 project, which has come under fire from both political sides, would link London to Birmingham by 2026, with tracks to cities in the north of England including Leeds and Manchester, built by 2030.
Once complete, journey times will fall by an hour from London to Manchester, by 53 minutes from London to Glasgow and by 65 minutes from Birmingham Curzon Street to Leeds.
However, the NAO said phase one is already running £204 million over budget and phase two, between Birmingham, Manchester and Leeds, is £7 billion over budget.
"HS2 is a large, complex and ambitious programme which is facing cost and time pressures," said Amyas Morse, head of the NAO.
"The unrealistic timetable set for HS2 Ltd by the Department [for Transport] means they are not as ready to deliver as they hoped to be at this point.
"The Department now needs to get the project working to a timescale that is achievable."
The NAO added that integrating HS2 into the wider UK rail network "is challenging and poses risks to value for money".
The company behind the project, HS2 Ltd, said it acknowledged that challenges remain but it is "on track to deliver its strategic scope and to do so on budget".
"The role of the NAO is to challenge projects such as HS2 and through that challenge, improve the way they deliver for the taxpayer," said the chief executive of HS2, Simon Kirby.
"This report does this and we accept that challenge. It also, however, recognises the real progress we have made in taking the concept of HS2 and moving it nearer reality."
Transport Minister Robert Goodwill said: "HS2 is on track and the National Audit Office agrees. We have strong cross party support and are on schedule to gain the powers needed to start building HS2, which the NAO acknowledges is a significant achievement."