MINISTERS have blocked the release of a report into the controversial HS2 rail project, over-ruling a decision by the freedom of information watchdog which said it should be disclosed.

Campaigners against construction of the high-speed line running through Oxfordshire are demanding the publication of a review of the £42bn scheme drawn up in 2011 for the Major Projects Authority (MPA).

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”.

Mr McLoughlin said in a statement to MPs: “The Major Projects Review was conducted to inform on 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, meaning the project had significant issues which would need to be addressed before work could go ahead.

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.”

Bicester MP Sir Tony Baldry said: “I am not entirely clear what has been the exact status of this report, but I have no doubt the members of the Bill committee will want to see as much as is relevant.”

Richard Houghton, of the HS2 Action Alliance, which opposes 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.”