Anti-HS2 campaigners have accused the government of censorship after the transport secretary blocked the release of a sensitive report believed to undermine the case for the proposed high-speed rail network.

Patrick McLoughlin took the unusual step of vetoing the publication of a November 2011 review by the Major Projects Authority, despite the information commissioner saying it should be released after Freedom of Information requests.

Opponents of the £42.6bn HS2 project believe that the report contained material that painted the scheme in scathing terms. The MPA review, one of a series conducted into major public spending infrastructure projects across the country, downgraded its assessment of HS2 to amber-red – signalling that the scheme's successful completion was believed to be in doubt.

However, the government has argued that releasing the report would undermine the decision-making process, as interviews were given by officials and employees with assurances of confidentiality in a "safe space".

McLoughlin said the decision to exercise the power of veto "was not taken lightly" and it was "an exceptional case" but claimed the move was to preserve future policymaking rather than suppress damaging information. He said that interviewees needed to know they could "speak with absolute frankness and candour" and the subsequent reports needed to be "drafted in frank, and if necessary blunt, language" so that "problems are confronted and addressed, and are not avoided or overlooked".

He told MPs: "In my view, there is nothing in the nature or content of this particular report which outweighs that strong public interest against disclosure."

However, Richard Houghton, of HS2 Action Alliance, 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.

"Independent research says the majority of the population does not want HS2, so why shouldn't the public know what the MPA has to say on its viability? 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."

Cheryl Gillan MP – one of the leading opponents in the Commons against the line which will link London to Birmingham, Manchester and Leeds – said she was disappointed and would seek to have the decision reversed. She said: "This decision can only lead to the speculation and conclusion that the report is unfavourable towards HS2."

A government spokesman said: "It's important to strike a balance between the benefits of transparency and protecting the ability of officials to 'speak truth to power'. We have already published project-level data in our annual report of major projects and have no plans to go further.

The information commissioner Christopher Graham said it was a disappointing . He said: "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."