Blocking the publication of a report into the risks of NHS reforms is a sign that ministers want to downgrade freedom of information laws, a watchdog has warned.
Information Commissioner Christopher Graham launched a scathing criticism of the decision to exercise the Government's veto in a report on the case to Parliament.
Health Secretary Andrew Lansley deployed it to block an Information Tribunal ruling that he should meet Labour demands to disclose the document.
Mr Graham dismissed his claims that there were exceptional circumstances and that it involved a matter of principle.
"The arguments are deployed in support of what is, in fact, the direct opposite of the exceptional - a generally less qualified, and therefore more predictable, 'safe space'," he wrote.
"As such, the Government's approach in this matter appears to have most to do with how the law might be changed to apply differently in future."
The November 2010 Transition Risk Register set out internal Government assessments of the risks posed by the reforms in the Health and Social Care Act, which became law in March after a tortuous passage through Parliament.
Labour MP John Healey, who was shadow health secretary when it was drawn up, called for the register to be published under the Freedom of Information Act - a demand backed by Mr Graham.
A Government appeal was then rejected unanimously by the Information Tribunal.
Mr Graham dismissed the argument put forward by ministers and senior civil servants that publication would have a "chilling effect" on officials' willingness to be frank.
There was "no evidence" to back the claim, he said - pointing to the previous release of a similar document concerning the introduction of ID cards.
And there was no reason to suppose it would set a precedent for other risk registers.
But the Cabinet agreed last week that the ministerial veto should be used.
In his report to Parliament, Mr Graham said the decision did not comply with the "statement of policy" regarding the use of the veto.
That sets out that it should be restricted to cases where disclosure would damage cabinet government or the constitutional doctrine of collective responsibility or where the public interest in maintaining those outweighed the public interest of publication.
"None of the criteria for 'exceptional cases' in the Statement of Policy are met in the present case," the Commissioner said in his report.
"Furthermore, the Commissioner does not consider that sufficient reasons have been given as to why this case is considered to be exceptional, particularly in light of the Tribunal's decision dismissing the Department's appeal.
"The Commissioner notes that much of the argument advanced as to why the case is considered to be exceptional merely repeats the arguments previously made to Commissioner and the Tribunal and which were in part dismissed by the Tribunal."
The only three previous uses of the veto all involved Cabinet material, he noted.
"The Commissioner would wish to record his concern that the exercise of the veto in this case extends its use into other areas of the policy process.
"It represents a departure from the position adopted in the Statement of Policy and therefore marks a significant step in the Government's approach to freedom of information."
Mr Healey indicated that he was considering a judicial review on the decision.
"This is the third time the Government's case for secrecy has been heard and dismissed. It's the third damning verdict on their desperate efforts to hide from the public the risks of their huge NHS upheaval," he said.
"The Information Commissioner confirms the Cabinet is overriding the law and their own Government policy with the political veto on the NHS risk register.
"In blunt terms, this report tells Parliament that ministers have ridden roughshod over existing FoI law and it warns that this case signals plans to change the law and roll back the public's right to know.
"The political veto has only ever been used before to protect Cabinet material. This move widens the span of Government information protected under veto. It is a landmark in the Executive's bid to draw a wider veil of secrecy over Government decisions.
"I have challenged the Cabinet's veto in the Commons, and I've have not ruled out a challenge in the courts."
The controversial use of the veto will be considered by an influential committee of MPs as part of a review of how freedom of information legislation is working in practice.
Mr Graham indicated in his report that he considered it an issue that should be included in the justice select committee's inquiry.
A spokesman for the cross-party committee said the MPs would examine the implications of the latest decision in a report due to be published later this year.