The government has blocked a 'warts and all' HS2 report claiming it is not in the public interest to release it.
According to the BBC, the 2011 Whitehall document is reportedly full of frank comments from people working on the scheme, who were encouraged to let rip even if it meant criticising bosses.
However, Cabinet Ministers took advantage of a rarely-used law - recently used to stop cabinet minutes on the Iraq war and private letters from Prince Charles to Camilla - to block its release.
Anti-HS2 campaigners are outraged. 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."
However, 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."
Campaigners have now applied for access to the report under Freedom of Information laws and had been supported in a ruling by the Information Commissioner, Christopher Graham.
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."
