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The HS2 rail line has been given an "unachievable" rating by the official infrastructure watchdog.
It has been granted a "red" warning for its first two phases - from London to Birmingham then onto Crewe - by the Infrastructure and Projects Authority.
Issues were raised related to budget, schedule and project definition.
The multi-billion pound project aims to connect London with central and northern England but has faced major delays and criticism.
The red rating, in the IPA's annual report on major projects, states: "Successful delivery of the project appears to be unachievable."
It adds there are "major issues with project definition, schedule, budget, quality and/or benefits delivery, which at this stage do not appear to be manageable or resolvable".
"The project may need re-scoping and/or its overall viability reassessed."
The phase of the project running from Crewe to Manchester was given an "amber" grading, meaning successful delivery "appears feasible" but that "significant issues already exist".
The London to Birmingham leg of HS2 was due to open in 2026, but is now expected between 2029 and 2033.
The second phase was due to open in 2032-33, but has been pushed back to 2035-2040.
An eastern leg of the line running to Leeds has been scrapped, and instead a shorter high-speed line will link Birmingham and East Midlands Parkway.
Management issues and unrealistic land valuations have also caused costs to spiral.
A budget of £55.7bn for the whole project was set in 2015, but the target cost excluding the eastern leg of Phase 2b from the West Midlands to the East Midlands has soared to between £53bn and £61bn at 2019 prices.
Earlier this month, HS2 Ltd's chief executive Mark Thurston announced his resignation - he will leave his role in September after six and a half years.
His resignation came as phase one of the project between London and Birmingham is under construction, with major work taking place at more than 350 sites.
In March, transport secretary Mark Harper announced work at London Euston would be paused for two years after costs increased from £2.6bn to £4.8bn.
A spokesperson for the Department of Transport said the government remains committed to delivering HS2.
"HS2 will bring transformational benefits for generations to come, improving connections and helping grow the economy," they said.
The IPA is the government's centre of expertise for infrastructure and major projects.