The health secretary Andrew Lansley has been accused of playing "dumb politics" and trying to "bury bad news" for vetoing the publication of a confidential risk assessment of the Government's contentious NHS reforms.
The move to defy an Information Tribunal ruling that the risk register should be released was agreed by the Cabinet on Tuesday 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, who along with Information Commissioner Christopher Graham and then the Information Tribunal, called for the register to be published under the Freedom of Information Act, denouinced the health secretary's veto.
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."
After the Cabinet agreed 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."
Shadow health secretary Andy Burnham also criticised the move, saying: "This disgraceful decision is a cover-up of epic proportions.
"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."
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 28 September 2010, and it is not known what changes were made before the completion of the transition risk register on 10 November.
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."
Dr Peter Carter, chief executive and general secretary 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.
"We believe it is wrong that yet again the government is refusing to publish, in full, the risks associated with these reforms.
"We are on record as agreeing with the Information Commissioner ruling that there was a strong public interest in the publication of the register and that it should be published as soon as possible.
"Today's decision is astonishing and means that the public are only being presented with a partial picture of the NHS reforms."
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