Ministers were warned 18 months ago of the risk that NHS reforms could lead to a loss of financial control, reduced productivity and emergencies being less well managed, according to a leaked document published on Tuesday morning.
The draft risk register was published on the day the Health and Social Care bill is expected to complete its tortuous passage on to the statute book by obtaining Royal Assent, more than 14 months after first being tabled in
the House of Commons in January last year.
The Department of Health, which refused to comment on today's leak, has resisted a ruling from the Information Commissioner that it should release the final version of the risk register in response to a freedom of information request from Labour.
It also resisted the findings of an appeal tribunal which also called on ministers to publish the document.
Shadow health secretary Andy Burnham said today's document showed that ministers were warned before they launched the Bill that it was "likely to cause major damage to the NHS".
Lansley and Burnham will clash in the Commons at health questions on Tuesday morning, where the leaked register is certain to come up.
The 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.
The Bill, which will devolve 60% of the NHS's budget to new GP-led consortia, has changed fundamentally since that date, with more than 1,000 amendments during its passage through Parliament.
Identifying 43 separate areas of potential risk, the draft register rates each on a scale of one to five, where a rating of one means little likelihood and very low impact and five means almost certain to occur and very high impact.
The likelihood and impact figures are multiplied together to give an overall risk rating, with a maximum score of 25.
Among 13 areas given a risk rating of 16 - with likelihood and impact each assessed at four out of five - were:
Other dangers, considered to have a lower rating of 12, included the risk that "NHS role in emergency preparedness/responsiveness is more difficult to manage through a more devolved organisation, and so emergencies are less well managed/mitigated".
Staff concerns and union action over the reforms could lead to "deterioration in relations, lower productivity in the
Department of Health/NHS and delays in programme", the document said.
And there was a warning that strategic health authorities and primary care trusts might lose "good people" who then have to be re-employed to run the new system.
Mr Burnham told the Guardian: "Now we know why David Cameron refused to publish the risk register before the Bill was through Parliament - it's because civil servants were telling him his reorganisation was likely to cause major damage to the NHS.
"David Cameron will never be forgiven for knowingly taking these risks with the country's best-loved institution."
Last week the Deputy Prime Minister told MPs that he had seen the final version of the risk register, and had no problem with it.
A Department of Health spokesman said: "We do not comment on leaks. We have always been open about risk and have published all relevant information in the impact assessments alongside the Bill.
"As the latest performance figures show, we are dealing with those risks, performance is improving - waiting times are down and mixed-sex wards are at an all-time low - and we are on course to make the efficiency savings that the NHS needs to safeguard it for the future."