A draft copy of a review into the HS2 high-speed railway linking London and the North of England says it should be built, despite its rising cost.
The government-commissioned review, launched in August, will not be published until after the election.
It says the project might cost even more than its current price of £88bn.
Members of the panel which produced the review have told the BBC that the draft recommends that HS2 should be built with only relatively minor alterations.
These include reducing the number of trains per hour from 18 to 14, which is in line with other high-speed networks around the world.
The document says that even the most controversial stretch of the railway - linking west London to central London - should go ahead.
Business leaders and politicians in the North of England have welcomed the review's preliminary findings.
But the draft does not have the support of the review's deputy chair, Lord Berkeley.
In a letter seen by the BBC, he criticised the review's "lack of balance" and said the cost of the scheme had not been properly scrutinised.
In the letter, sent to Doug Oakervee, the chairman of the review panel, Lord Berkeley said about the review: "I cannot support its conclusions or recommendations.
"My concerns are about the process of the report's preparation and its outcome.
"We had to complete the work in a very short time. I also detected a trend in may of the discussions within the review to accept that HS2 will go ahead.... rather than look at the pros and cons of alternative options.
"I reserve the right to publish my own alternative report in due course."
Mr Oakervee said he regretted that Lord Berkeley "feels unable to give his support."
"He participated fully in panel discussions that have seen all other members converge their views, based on the extensive evidence considered," Mr Oakervee added.
A report in The Times says that the review found that without HS2, "large ticket price rises" would be needed to discourage people from travelling at peak times.
Henri Murison, director of the Northern Powerhouse Partnership, said: "The Northern Powerhouse Independent Review on HS2 said that there were no identified credible alternatives to HS2 in order to deliver the same capacity, and that it has the potential to unlock greater growth in the North and Midlands.
"It is welcome that their recommendations are mirrored by the government's own Oakervee Review."
However, Penny Gaines, chairwoman of the Stop HS2 campaign, said: "HS2 was a bad project when it was originally announced and was supposed to cost £33bn, it was a bad project when it was supposed to cost £55bn and it is a bad project now the cost is expected to be more than £88bn.
"It should be cancelled as soon as possible, so the government can focus on the real transport priorities."