Health Secretary Andrew Lansley vetoed the publication of a confidential risk assessment of the Government's contentious NHS reforms today.
The move to defy an Information Tribunal ruling that the risk register should be released was agreed by the Cabinet this morning.
Mr Lansley said he believed in "greater transparency" but that it was also essential to retain "a safe space where officials are able to give ministers full and frank advice in developing policies and programmes".
The November 2010 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 called for the register to be published under the Freedom of Information Act - a demand backed by Information Commissioner Christopher Graham and then the Information Tribunal.
But the Cabinet agreed today that the "ministerial veto" should be used to prevent publication.
Mr Lansley said: "Had we not taken this decision, it is highly likely that future sensitive risk registers would turn into anodyne documents, and be worded quite differently with civil servants worrying about how they sound to the public rather than giving ministers frank policy advice."
He said he was instead publishing a document setting out "key information" from the register but protecting its "language and form".
"This is not a step I have taken lightly. I am a firm believer in greater transparency and this Government and this department have done far more than our predecessors in publishing information about the performance and results of our policies.
"But there also needs to be safe space where officials are able to give ministers full and frank advice in developing policies and programmes.
"The Freedom of Information Act always contemplated such a 'safe space' and I believe effective government requires it.
"That is why Cabinet has today decided to veto the release of the department's transition risk register."
Mr Lansley said the public had "all the information necessary to understand what we considered the risks to be and how we have acted to mitigate them".
"To continue to be transparent about the risks we considered, and to be equally clear about how we have mitigated those risks, I have also published today a document setting out key information relating to the areas of risks in the original risk register, how we have met those risks head on and how we will continue to do so," he said.
Mr Healey said the decision was "poor policy and dumb politics", and would "only fuel doubts and distrust" about the reforms.
The veto is used very rarely - the last Labour government vetoed the release of Cabinet minutes relating to the invasion of Iraq.
"This is a desperate act which will backfire badly. It is an admission of defeat on the legal arguments for public release," Mr Healey, a former shadow health secretary, said.
"It is totally over the top to place NHS changes on the same footing as preparations for the Iraq war.
"There must be some very big risks in the Government's NHS reorganisation for ministers to override the law with their political veto.
"Ministers have made the announcement in the very last hour of the last day, trying to bury this bad news on the eve of the Queen's Speech.
"The Government has lost twice in law, yet still won't accept that patients and NHS staff have the right to know the risks ministers are running with the biggest ever NHS reorganisation."
A draft risk register leaked in March showed that ministers were warned of the risk that the reforms could lead to a loss of financial control, reduced productivity and emergencies being less well managed.
That document was produced on September 28, 2010, and it is not known what changes were made before the completion of the transition risk register on November 10.