The delivery of HS2 has been given the highest risk warning by Government experts on major projects.
It is the first time the Infrastructure and Projects Authority (IPA) has assigned the high-speed railway with a red rating since its annual reports began in 2013.
The IPA assesses the likelihood of large schemes fulfilling their objects on time and in budget.
Red ratings are given to schemes for which “successful delivery of the project appears to be unachievable”.
Classifications in the latest annual report were based on a snapshot taken in September 2019 and do not take into account the impact of the coronavirus pandemic.
The Government-commissioned Oakervee Review warned last year that the final bill for HS2 could reach £106 billion at 2019 prices.
Despite it running tens of billions of pounds over budget and several years behind schedule, Prime Minister Boris Johnson gave the green light for the railway in February 2020.
The project was given a revised budget and schedule as part of his decision.
Two months’ later, ministers gave the go ahead for it to enter the construction phase.
That meant HS2 Ltd, the Government-owned company developing the high-speed railway, was given approval to issue a notice to proceed to firms contracted to design and construct bridges, tunnels, embankments and viaducts for phase one between London and Birmingham.
A Department for Transport spokesman said: “This report refers to the status of the HS2 project in September 2019, before it was comprehensively reset in February 2020, so does not take into account the intervening period up to the present.
“We now have stricter controls over the project including a dedicated HS2 minister, and are confident HS2 is being delivered with the strict oversight, accountability and transparency needed.”
A spokesman for HS2 Ltd said: “The ongoing preparatory work undertaken by HS2 Ltd in the past 12 months now mean that the cost and schedule estimates for Phase One are the most robust they’ve ever been.”
“HS2 is already creating thousands of jobs, apprenticeships and contracts for SMEs (small and medium-sized enterprises) across Britain, and will play a major role in the country’s post-pandemic economic recovery.”
Richard Houghton, director of protest group HS2 Action Alliance, claimed the railway’s business case “has always been based on voodoo economics”.
He went on: “With future use of trains likely to dramatically reduce as fewer workers commute to city centre offices, it really is time for the Government to listen to its own reports, cancel HS2 and spend the funds on local transport and high-speed broadband.
“This will do far more, far quicker, to deliver on its economic promise of levelling up.”