The chairman of HS2 is to step down after the Government said that the estimated £66 billion cost of the rail project was not an "accurate or reliable" figure.
Sir Jon Thompson, who said last month that a "bat shed" covering 900 metres of HS2 tracks near Aylesbury would cost £100 million to build, announced he will leave his role in the spring.
It came as Heidi Alexander, the Transport Secretary, said in a progress report published on Tuesday that there was doubt over forecasts that the cost had spiralled to between £54 billion and £66 billion - up to double the original estimate of £33 billion.
The report warned that the estimate included parts of the since-cancelled Euston station project. It added: "As it doesn't take into account this work, or factor in private financing for Euston - which this government is committed to securing - we do not think these figures are accurate or reliable.
"It remains highly uncertain and subject to further assurance and has not been approved by the HS2 Ltd Board or [The Department for Transport] ... but has been included here for transparency."
The £66 billion estimate, which does not take inflation into account, is based on costings from 2019.
The report said the project's spiralling budget had been caused by factors including "environmental and planning compliance, as well as the disruption caused by external factors, primarily Covid and the Ukraine conflict."
So far, £32.8 billion has been spent on HS2 since it was approved in 2012, and the latest estimates come following years of warnings that its spending was out of control.
In 2020 the Public Accounts Committee said the Department for Transport (DfT) had "failed to provide Parliament with clear warning that the programme was going off-course and value for money was at risk".
Rachel Reeves, the Chancellor, said in October that HS2 would be built to Euston, ending uncertainty brought about by Rishi Sunak axing public funding for the line's final 4.5-mile stretch into central London.
Instead, both the required tunnels and the extension to Euston station necessary to serve HS2 would have to be privately funded, the Conservative former prime minister said last year.
Sir Jon's departure from HS2 comes after a new chief executive was appointed to the beleaguered rail project in May.
Mark Wild, a former boss of Crossrail, took up the post this month. The report said: "We have made it the number one priority of the new chief executive, after safety, to drive costs down."
Sir Jon is expected to become the new chairman of the company behind Sports Direct, businessman Mike Ashley's Frasers Group, Sky News reported in November.
A spokesman for HS2 Ltd said that "clearly changes in delivery are needed" for the project to "realise its full potential".
They added: "Mark Wild joined as our new chief executive this month and clearly recognises the programme's serious cost challenges.
"He is now leading a comprehensive review of HS2's cost and schedule which will report into government next year and lead to a full reset of the project."
A DfT spokesman said of the £66 billion costings: "These estimates are out of date and don't take into account the urgent measures this government took to get the project back under control or the previous government's decision to cancel Phase 2 and attempt to secure private funding for Euston.
"The Transport Secretary has asked HS2 to urgently provide updated and clearer estimates of the project as soon as possible and we have made it the number one priority of the new chief executive to drive costs down."