Anti-HS2 campaigners have suffered a setback after ministers blocked a report into the controversial high-speed rail scheme.
The 2011 Whitehall document is a warts-and-all analysis of how HS2 was progressing at the time.
It is reportedly full of frank comments from people working on the scheme, who were encouraged to let rip even if it meant criticising bosses.
The government said it was not in the public interest to release the paper.
Ministers took advantage of a rarely-used law to block its release.
Campaigners had applied for access to the report under Freedom of Information laws and had been supported in a ruling by the Information Commissioner.
The government used a rare veto - seen as the nuclear option in this sort of case - to keep the report under wraps.
It is a veto that was last used to stop publication of cabinet minutes on the Iraq war and private letters from the Prince of Wales to ministers.
Ministers argue that they were all promised those comments would never be made public and say that publishing the report would undermine future reviews because staff would be more guarded.
A spokesperson for the government said: "The Major Projects Authority will not be truly effective if officials fear that their frank advice to ministers could be disclosed.
"We have already published project-level data in our annual report of major projects and have no plans to go further.
"The government has decided that it is not in the public interest to release this report."
The government also points out that the information is more than two years out of date, and that the project is on a much surer footing these days.
But campaigners have accused them of hiding bad news.
Richard Houghton of HS2 Action Alliance, the biggest national HS2 opposition campaign, said: "If the MPA report was supportive, then it would have been published like a shot.
"Secrecy and withholding of information kills governments - especially in an era when public and business alike are deeply suspicious of politicians.
"That is something which should be carefully considered, not just by the current government but any likely future Labour-led government."
The information commissioner, Christopher Graham, called it a "disappointing decision".
He said: "I'll be studying the secretary of state's explanation to understand why this has been ruled an exceptional case.
"There's important legal issues to be considered here, and I'll be highlighting our view of them in an open letter to the justice committee in due course."