Ministers have ditched the official £56 billion price tag for HS2 after Boris Johnson said the scheme’s final cost could amount to double the formal estimate.

Mr Johnson’s regime has broken away from the insistence of Theresa May’s government that the rail line will fall within the £55.7 billion budget set in 2015.

The moves comes after several senior ministers under Mrs May privately warned that the scheme, due to stretch from London to Birmingham and then on to Leeds and Manchester, was on course to massively overshoot its budget. Last month it emerged that the new chairman of HS2 Ltd, the government-owned firm building the line, believed that the final costs could rise to between £70 billion and £85 billion.

Asked about the shift in the official stance under Mr Johnson, a Government source said: "It is important to be clear-eyed about how much this is going to cost the taxpayer."

As recently as last month, while Mrs May was still Prime Minister, Nusrat Ghani, her HS2 minister, told the Commons: "I stand here to state confidently that the budget is £55.7 billion." Chris Grayling, who was sacked as Transport Secretary by Mr Johnson, had also insisted that the budget would not rise, saying: “I am very clear on HS2; it’s got a budget and it’s got to live with that budget."