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The unpublished report, seen by the Financial Times, says there is "considerable risk" that estimated costs could rise by another 20%.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 - a branch of which would run from London through South Yorkshire to Leeds, with a branch running into North Yorkshire - 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".