Shadow health secretary Andy Burnham has accused ministers of a "cover-up of epic proportions" by deciding to veto an Information Tribunal order to publish the NHS risk register.
In a Commons statement Health Secretary Andrew Lansley said publication was blocked because governments needed a "safe space" to develop policy.
He told the Commons that it could have led to "misinterpretation and misuse".
The document assesses risks relating to the recently passed NHS shake-up bill.
Mr Burnham said the decision was "a major step backwards to secrecy and closed government" and broke with a precedent set by the previous Labour government.
He argued that the reason the document had not been made public was because the government knew "the public would never forgive them if they could see the scale of risk they are taking with the National Health Service".
In March, the Information Tribunal ordered publication of the risk register document, which was drawn up by officials and lists the threat to the delivery of services from any changes.
A leaked 2010 draft of the register warned that the cost of GP care could rise, responses to health emergencies could worsen and there was a high chance that managers might lose financial control of the NHS.
The tribunal ruled that the public interest in publishing the final document was "very high, if not exceptional".
But the cabinet decided on Tuesday to veto the ruling.
Defending the use of the publication veto - which has only been used by ministers three times before - Mr Lansley told MPs the decision was about "long term principles and good government".
"If such registers are disclosed at sensitive times in relation to sensitive issues - as would have been the case here - it is highly likely they would have been open to misinterpretation and misuse."
He pointed out that the government had published a document which set out some of the risks contained in the register - and set out how they had been mitigated against during the parliamentary passage of the Health and Social Care Act.