The HS2 rail line has been given an “unachievable” rating by an official watchdog.
It has been given a “red” warning for its first two phases – from London to Birmingham then onto Crewe – by the Infrastructure and Projects Authority.
The project aims to create high-speed rail links between London and central and northern England, but has faced major delays and criticism.
The government says it remains committed to delivering HS2.
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.