Building the high-speed rail link HS2 could cost up to £106bn, a government-commissioned review has said.
The unpublished report, seen by the Financial Times, says there is “considerable risk” that estimated costs could rise by another 20%.
In 2015, HS2 was set to cost £56bn.
The review also recommends pausing the second phase of the project while experts look at whether conventional lines could help link Birmingham to Manchester and Leeds instead.
Some £8bn has already been spent on the project, which will connect London, the Midlands and northern England using trains capable of travelling at 250mph.
BBC transport correspondent Tom Burridge said that “on balance”, the review recommends the government should proceed with the project, but he added this “is hardly a ringing endorsement”.
On Friday, Transport Secretary Grant Shapps said a decision on whether to go ahead with the project would be made “very soon”.
The government previously promised to make a decision before the end of 2019.
£7.4bnAlready spent on the project
9,000Jobs supported by the railway
345 milesNew high-speed track
50 minutesJourney time saving between London and Manchester
The review, led by former HS2 chairman Doug Oakervee, comes as around 15 Conservative MPs from across England prepare to meet the prime minister this week to raise significant concerns about the project.
Some will press him to abandon the rail link in favour of other infrastructure spending in the north of England.
Boris Johnson has sent varying signals on the project so far, insisting costs cannot continue to spiral while also hinting that, temperamentally, he is in favour of a scheme that could help rebalance the UK’s lopsided economy and address regional disparities.
In September, a “stocktake” report by the chairman of HS2, Allan Cook, concluded that the cost had risen to between £81bn and £88bn.
The first segment of project between London and Birmingham is due to open at the end of 2026, with the second phase to Leeds and Manchester expected to be completed by 2032-33.
The spending watchdog, the National Audit Office, will publish another report about HS2 in the coming days.
Despite concerns about the rail link, which is Europe’s largest infrastructure project, work is not on hold and the project currently gets through around £250m a month.
What will HS2 do?
HS2 is a new railway line which, once completed, would run from London to the West Midlands, Manchester and Leeds.
Trains on the London to Birmingham route would be 400m-long (1,300ft) with up to 1,100 seats. They would run as many as 14 times per hour in each direction.
The Department for Transport says the project will cut Birmingham to London journey times from one hour 21 minutes to 52 minutes.
Once the second phase is complete, Manchester to London journeys would take one hour seven minutes (down from two hours seven minutes), and Birmingham to Leeds would take 49 minutes (down from two hours).
This would effectively reduce journey times between London and Edinburgh and London and Glasgow by an hour, to three-and-a-half hours.
The government hopes its creation will free up capacity on overcrowded commuter routes.