Britain’s new high-speed rail line will be the most expensive railway in the world with costs per mile expected to reach £403m, according to Government calculations. 

The HS2’s first phase between London and Birmingham will cost almost £48bn, according to expert analysis commissioned by the Department for Transport (DfT). 

Michael Byng, who created the method used by Network Rail to cost its projects, made the estimates for DfT and said the line would cost double the official figure and 15 times more than the cost per mile of the TGV in France, according to the Sunday Times.

The scheme could cost up to £104bn in total, including extensions to Manchester and Leeds, he believes. The first 6.6 miles from Euston to Old Oak Common would cost £8.25bn, or £1.25bn a mile.

The senior civil servant responsible for the project reportedly told Mr Byng he was “very worried” the official costs could be unrealistic.

Former transport minister John Spellar said the controversial project was an “ever-deepening bottomless pit”.

“HS2 has not questioned the figure, or my methodology, nor have they come up with any structured estimate of their own,” Mr Byng told the Sunday Times. 

“A couple of days later I got a call from the DfT saying ‘Look, if 6.6 miles is going to cost us £8.25bn, what chance have we got of getting to Birmingham?’ I said: ‘I’ll work it out for you.’

“Michael Hurn, the project sponsor at the DfT, is a very good guy and is very worried at the advice he’s been given [by HS2]. The big contractors are also worried. They’ve said when they submit a bid it’s nowhere near [as low as] the estimates that HS2 have got for the job.” 

HS2 said it “did not recognise” Mr Byng’s figure and was “confident we will deliver the project on time and on budget”. 

The DfT said: “We are keeping a tough grip on costs and the project is on time and on budget.”

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It comes after Chancellor Philip Hammond got the cost wrong by at least £20bn in a radio interview.

Mr Hammond said HS2 will cost the taxpayer £32bn, when in fact it is expected to cost more than £52bn.

The Chancellor had been attacking Labour's policy costings, which he claimed “don't add up”, when he made the ironic gaffe.

The project has also been mired in delays and complications. The engineering firm that was handed the multimillion-pound contract to develop HS2 pulled out of the project less than two months after it was selected to complete the work.

The Government confirmed on 9 February that it had appointed US-based company CH2M to help develop a section of the network north of Crewe to Manchester and from the West Midlands to Yorkshire.

But in March, CH2M said it had decided to withdraw its interest, following what it called “continuing discussions” between HS2 and CH2M over the award of the contract.