Ministers have blocked the release of a report into the controversial HS2 rail project by over-ruling a decision by the freedom of information watchdog that it should be disclosed to campaigners.
Campaigners against construction of the high-speed line between London and the north of England are demanding the publication of a review of the £42 billion scheme drawn up in 2011 for the Major Projects Authority (MPA).
They were hopeful that the Information Commissioner's decision last year to back their freedom of information request would lead to the revelation of damning information about doubts over the scheme's value for money.
But Transport Secretary Patrick McLoughlin said he was taking the "exceptional" step of vetoing the Information Commissioner's ruling that the project assessment review should be released, arguing that there was a "strong public interest against disclosure".
Announcing his decision in a statement to MPs, Mr McLoughlin said: "The Major Projects Review was conducted to inform the development of the HS2 project. The public interest in ensuring that projects of this scale, importance and cost are properly controlled and overseen is very high indeed.
"The assurance of confidentiality is important in the conduct of the review. In my view, there is nothing in the nature or content of this particular report which outweighs that strong public interest against disclosure."
The MPA has carried out reviews of dozens of high-value Government schemes to assess the effectiveness with which they can be delivered, and none of them are ever intended for publication. In 2013, the authority released a traffic-light rating for each of the schemes, which gave HS2 an "amber/red" assessment.
A Government spokesman said: "This Government is proud to lead the world on transparency and last year we published the first ever Major Projects Authority report which included information on all major projects.
"Hard-working people rightly expect the Government to keep tight control over how their taxes are spent on major projects and that's just what the Government's Major Projects Authority is working with departments to do.
"It's important to strike a balance between the benefits of transparency and protecting the ability of officials to 'speak truth to power'. 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."
Information Commissioner Christopher Graham said: "This is a disappointing decision, and 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."