A plan to bring high speed rail across England, connecting cities like Leeds and Sheffield to London at 225mph could go as much as £30billion over budget.
The chairman of the high-speed railway project has written to the Department for Transport to warn that it cannot be completed for the original £55.7 billion budget, the Financial Times says.
The newspaper said Allan Cook, who is reviewing the project amid cost concerns, has predicted the final figure could end up being between £70 billion and £85 billion.
The potential hike in price is down to various factors including engineering costs, poor ground conditions and extra costs due to it being designed to run faster than other comparable rail projects, the paper quoted an unnamed source close to the project as saying.
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HS2 is planned to run up to 18 trains per hour at a top speed of 225mph - faster than France's high speed TGV service which currently runs at around 200mph.
A spokesman for HS2 Ltd said: "We don't comment on leaks or speculation. We have previously noted that our chair, as you would expect, continues to scrutinise the programme, and regularly reports back to the Department.
"We are determined to deliver a railway that rebalances the economy, creates jobs, boosts economic growth and is value for money for taxpayers."
A spokesman for the Department for Transport said: "The chairman of HS2 Ltd is conducting detailed work into of the costs and schedule of the project to ensure it delivers benefits to passengers, the economy and represents value for money for the taxpayer.
"This work is ongoing. We expect Allan Cook to provide his final assessment in due course."
It is not the first time concerns have been raised around the project coming in on budget.
A report by the House of Lords Economic Affairs Committee in May warned it was "far from convinced" it would be built for the original planned cost, and former HS2 chairman Sir Terry Morgan has claimed the project cannot be built to its current specification on budget.
Meanwhile, a separate report by the New Economics Foundation (NEF) think tank argued earlier this year that the project will widen the north-south divide and should be scrapped in favour of significant investment in rail services outside London.
Tory leadership candidate Boris Johnson has already said he will have a review of the project should he become the UK's next prime minister.
Mr Johnson said the project is going to cause "a great deal of difficulties" for his constituents in Uxbridge and South Ruislip.
Phase 1 of HS2 is planned to run between London and Birmingham from December 2026, while a second Y-shaped phase will launch in two stages.
Phase 2a from the West Midlands to Crewe is due to open in 2027, followed by Phase 2b from Crewe to Manchester, and Birmingham to Leeds, in 2033.
HS2 trains will also serve destinations on conventional lines beyond the new high-speed network, including York, Newcastle, Liverpool, Glasgow and Edinburgh.