The risk assessment of the NHS overhaul in England will not be published after ministers vetoed demands to release it.
The cabinet decided that releasing the document listing possible risks to services could harm the quality of future advice from civil servants.
Labour's shadow health secretary Andy Burnham accused the government of a "cover-up of epic proportions".
An Information Tribunal had ruled in March that the risk register should be published.
The risk register is a written document drawn up by policymakers that lists the threat to the delivery of services from any changes.
The NHS risk register was put together two years ago at the same time the white paper outlining Health Secretary Andrew Lansley's plans was compiled.
A draft version of the register was leaked, revealing the risks of rising costs of GP care, poorer response to health emergencies and the high chance that managers might lose financial control of the NHS.
The Information Tribunal had ruled that the public interest in publishing the risk register was "very high, if not exceptional".
But, announcing the veto, Mr Lansley said: "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."
Despite blocking the release of the information, the government has published a document which sets out some of the risks and how they have been mitigated against during the parliamentary passage of the Health and Social Care Act.
Mr Burnham said: "David Cameron is desperate to keep the NHS risk register secret because he knows that, if people could see the scale of the risks he is taking with the NHS, they would not forgive him.
"All the warnings from doctors, nurses and patients about the dangers of his re-organisation were true and echoed in private by civil servants but the prime minister chose to ignore them."
Dr Peter Carter, head of the Royal College of Nursing, said: "Despite overwhelming opposition, the government is undertaking the most radical restructure of the NHS at the same time as the service faces severe financial challenges."
The refusal to publish the risk assessment was "astonishing", he said, arguing that "the public are only being presented with a partial picture of the NHS reforms".
The Information Tribunal considered the matter after the government had appealed against the information commissioner's earlier ruling that the register should be published.
An Information Commissioner's Office (ICO) spokeswoman said: "We will need to study the secretary of state's statement of reasons for imposing the ministerial veto in this case. These must, under the criteria established by the government, be 'exceptional'.
"We will present the commissioner's formal report on the matter to Parliament next week."
It is thought unlikely the commissioner can challenge the government's veto.
Publication had originally been sought in a freedom information request by John Healey MP, a former Labour health spokesman who called the government's latest move a "desperate act".
He added: "This decision will only fuel doubts and distrust about the government's NHS plans, as people rightly ask: 'What are they hiding from us?'"
The Campaign for Freedom of Information said: "We think the government should appeal against decisions that it dislikes, not veto them.
"The tribunal found that disclosing the register would have helped the public understand the risks and judge whether the government had properly addressed them.
"The government has turned that on its head. It has now published a detailed account of the action it has taken to address possible risks, but refused to say what those risks are - that means the public still can't judge."