Andrew Lansley has been told to reveal the government's own analysis of the risks that his shake-up of the NHS poses for the health service after a year-long battle to keep the study secret.

The information commissioner, Christopher Graham, has ordered the health secretary to publish the Department of Health's own risk assessment of the potential pitfalls involved in his radical restructuring of the NHS in England.

Public interest in information about the coalition's NHS plans was more important than ministers' insistence that disclosure would impede the formulation of government policy, Graham ruled.

"Disclosure would significantly aid public understanding of risks related to the proposed reforms and it would also inform participation in the debate about the reforms," he decided.

Critics of the plans say the document may prove to be highly significant and contain new revelations that could yet affect the passage through parliament of Lansley's controversial health and social care bill, which has reached its committee stage in the House of Lords.

Graham has upheld complaints by both John Healey, who was Labour's shadow health secretary until last month, and London's Evening Standard newspaper, about the DoH rejecting their separate requests under the Freedom of Information Act to see whatever assessment it had made of the risks.

The DoH twice broke the law in its refusal to acceded to the MP and newspaper's requests for the information, Graham has ruled. "The year-long coverup is a disgrace, especially when doctors, nurses, patients groups and the public are all so worried about the Tories' NHS plans. The commissioner's report is a demolition job of Lansley's attempts to keep the truth from the public," said Healey.

In his 14-page decision notice relating to Healey's complaint, Graham said that the MP had last year used the FoI Act to ask the DH for "a copy of the risk registers or risk assessments related to government plans regarding the modernisation of the NHS and the health and social care bill". The DH refused under the Act's section 35(1)(a) and said that revealing such information would interfere with policymaking.

However, "the commissioner has now investigated the complaint and found that section 35(1)(a) is engaged but that the public interest in maintaining the exemption does not outweigh the public interest in disclosure". The DH has 35 days from the date of the notice, 2 November, to publish its analysis.

The DH breached both section 1(1)(b) of the Act "by refusing to disclose the information to the complainant" and also section 17(1) because it did not tell Healey within 20 days of receiving his request that it was relying on section 35(1)(a) as its grounds for withholding the material.

Failure to publish the risk analysis within the 35 days "may result in the Commissioner making written certification of this fact to the High Court … and may be dealt with as a contempt of court," the Information Commissioner's Office's decision notice adds. But either side can appeal against the decision.

The DH would only say: "We are currently considering the decision notice from the Information Commissioner." But it had argued that disclosure could "deter from full, candid and proper deliberation of policy formulation and development", an argument that the ICO rejected.

Dr Clare Gerada, chair of the Royal College of GPs and a leading opponent of the government's NHS plans, said: "The RCGP, among others, have been concerned for a year about the risks associated with the DH's plans to 'liberate' the NHS; risks such as the increased costs involved, fragmentation of services, and widening of health inequalities – all things that poll after poll has showed that health professionals are worried about. I'm looking forward to seeing what this document says because it could vindicate people like myself who have been speaking out for patients for a year now."