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."
Richard Houghton of the HS2 Action Alliance, which is opposing the new rail link, said: "So far as we can see, the last time secrecy laws of this nature were invoked was during the Iraq war. The implications are immense.
" We obviously have not had access to the MPA report into HS2 although, as with everything of this nature, there are leaks and rumours which lead us to believe that the report is damning and could see heads rolling in both the Department for Transport and other sections of government.
"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."
Paul Thornton, who brought the original freedom of information request, said the issue was likely to go to judicial review.
He said the ministerial veto applied only to internal Government documents, but the review had been shared with HS2 Ltd, which is a company wholly owned by the Department for Transport with a remit to design and promote the new rail link.
"The veto can only be used when there is a dispute over public interest, and that cannot be used in this case because the report is not an internal communication," said Dr Thornton.
"This is a delaying tactic on the part of Government as they know decision is likely to go to judicial review. This should not be necessary as Parliament should be scrutinising this arbitrary decision and ensuring that it is overturned, as it creates an alarming precedent."
Stop HS2 campaign manager Joe Rukin said: "It is absolutely disgraceful that the Government doesn't want MPs, who should be fully informed before voting on the colossal expenditure HS2 entails, to actually have all the information about the project.
"This just proves that there has been, and continues to be, an agenda from Government to deceive MPs and the public with spin and dismiss any independent information showing just how bad the HS2 project is.
"It also shows that this Government's supposed 'transparency agenda' is as transparent as a lump of coal. What is the point in having reports on projects if they cannot be used to inform the debate around them?"