The HS2 rail programme has been given an "unachievable" rating by the official infrastructure watchdog.
A 'red' rating was assigned to the plans for the construction of the first two phases of the troubled rail line, from London to Birmingham and Crewe, by the Infrastructure and Projects Authority (IPA).
The red rating, contained in the IPA's annual report on major projects released last week, states: "Successful delivery of the project appears to be unachievable.
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"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."
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Construction on the first phase of HS2 between London and Birmingham is well under way, with four new stations being built along the route. Phase 2A will connect Birmingham up with Crewe.
Earlier this month, HS2's chief executive Mark Thurston announced his resignation amid major delays and cost pressures for the high-speed railway project.
He is to leave his role in September after six-and-a-half years leading the Government-owned company.
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Phase one of HS2 was initially scheduled to open in 2026 but this has been delayed to between 2029 and 2033 due to construction difficulties and rising costs.
A budget of £55.7 billion 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 £53 billion and £61 billion at 2019 prices.
In March, Transport Secretary Mark Harper announced work at Euston would be paused for two years as costs had ballooned to £4.8 billion compared with an initial budget of £2.6 billion.
Mr Thurston's resignation announcement came with phase one of the project at peak construction, with major work taking place at more than 350 sites. he IPA is the Government's centre of expertise for infrastructure and major projects.
It sits at the heart of government and reports to the Cabinet Office and HM Treasury.
A Department for Transport spokesman said: "Spades are already in the ground on HS2, with 350 construction sites, over £20 billion invested to date and supporting over 28,500 jobs.
"We remain committed to delivering HS2 in the most cost-effective way for taxpayers. HS2 will bring transformational benefits for generations to come, improving connections and helping grow the economy."