The peer had said that, despite backing the plans when Labour was in power, said that he now believed the line to be an “expensive mistake”.
Mr Johnson said: “That is why the Treasury is starting to panic, and the word around the campfire is that Lord Mandelson is actually doing the bidding of some fainthearts in Whitehall who want to stop it now – not the first or second Lords of the Treasury, clearly, but the beancounters.”
Mr Johnson criticised “the whole nightmare of consultation and litigation – and the huge army of massively expensive and taxpayer-financed secondary activities”.
This meant that £1 billion is likely to have been spent on HS2 before a sod of earth was cut.
He said: “It is the environmental impact assessments and the equalities impact assessments and the will-sapping tedium and cost of the consultations.
“Did you know that in order to build HS2 we are going to spend £1billion by 2015 – and they won’t have turned a single sod in Buckinghamshire or anywhere else?
“That is a billion quid going straight down the gullets of lawyers and planners and consultants before you have even invested in a yard of track.
“To understand the prohibitive costs of UK infrastructure, you need to take this haemorrhage of cash to consultants, and then multiply it by the time devoted to political dithering.”
The new estimate comes less than a fortnight after transport secretary Patrick McLoughlin said the projected cost has risen from £33billion to £42.6billion because of a “contingency2 fund to cover the cost of potential problems with the programme.
The Department for Transport said there were significant changes to the line, such as more tunnels to avoid disturbing the environment and those living near the line.
Labour leader Ed Miliband, his shadow chancellor Ed Balls and former transport secretary Lord Adonis are all said to still publicly back HS2.
Current shadow transport secretary Maria Eagle recently branded the project “essential” for Britain’s railways to cope with rising demand.
But the admission by Lord Mandelson has added to the growing fears that the political consensus over HS2 is in danger of being shattered.
The news came as a senior minister suggested that the government should scrap HS2 and spend the money on other transport projects instead.
David Lidington, the Europe minister, whose Aylesbury constituency in Buckinghamshire will be severely affected by the proposed railway from London to Birmingham, has written to Patrick McLoughlin, the transport secretary, detailing concerns over the new route.
Mr Lidington is known to have serious reservations, but his letter is the closest he or any government figure has come to calling for it to be scrapped altogether.