Health secretary Andrew Lansley looks more determined than ever not to reveal the findings of a risk assessment done on the government's NHS shakeup.

Lansley won the support of MPs, who voted on Wednesday by a majority of 53 against a Labour motion that the Department of Health should make its document public. However, growing disquiet among some Conservative MPs and Liberal Democrats was voiced by Lib Dem MP John Pugh, who told the often bad-tempered debate that the bill was "toxifying the Tories" and "sadly detrimental" to his party.

Lansley suggested to MPs that he might refuse to release the risk register even if instructed to do so by a tribunal due to meet in a fortnight to judge on his dispute with the information commissioner, who has instructed him to publish.

Lansley twice refused the opportunity to tell MPs he would accept the tribunal's judgment. Answering deputy Lib Dem leader Simon Hughes, the health secretary instead quoted from an article in the Observer by the information commissioner, in which Christopher Graham said he was "not infallible".

"The government has the right to appeal to the tribunal [following the information commissioner's ruling] and the tribunal is the proper place for that public interest test to be tested," he said.

Defending his decision, Lansley said the prospect of publishing such assessments reduced the quality of advice to ministers, meaning documents would become "bland and anodyne" and "cease to be of practical value".

"To be effective, a risk register requires all those involved to be frank and open about potential risk," Lansley told MPs. "It is their job to think the unthinkable and look at worst-case scenarios. It is vital nothing is done to inhibit that process."

Asked whether Lansley's comments suggested local and regional NHS risk registers, which have been published, were not as strong as they could be, a department spokesman said they could be "potentially watered-down".

As the Guardian reported last week, the risk assessments of the four English regional strategic authorities suggest there are wide-ranging concerns, including that patient care and safety could be damaged and costs could rise.

Lansley cleared up some confusion about the hotly debated risk register, saying the document in question was the "transition risk register", relating specifically to the reorganisation set out in the health bill, an assessment which was first drawn up in 2010 but is continually "reviewed and updated". This was different to the department's "strategic" risk register of all its operations.

The department said that, unlike the strategic authorities' and other NHS risk assessments, its risk register concentrated on policy development. However, a spokesman said refusal to publish the register extended also to explaining what questions it might cover, for example, if it dealt with how the bill might pass through parliament, or gave technical details about how its parts might impact on each other.

Labour's opposition day debate was fronted by

After the vote, Andy Burnham, the Labour shadow health secretary, said: "It is clear they are going to try to fight it: they are going to go to the High Court, go all the way, to go beyond the Bill [passing]."

He had insisted beforehand that MPs and peers had a right to know the implications of health reforms before they voted on the bill, which is currently in the report stage in the House of Lords.

"He [Lansley] is running unacceptable risks," said Burhnam. "What he's doing is wrong and needs to be stopped."

Burnham had to fend off repeated charges by Conservative MPs that he had refused similar requests for risk registers when he was health secretary in the previous Labour government. Burnham said he had refused to publish a different document – the strategic register – and that he had not been overruled by the Information Commissioner. Labour did release a similar policy-specific risk assessment, into Heathrow's third runway, when it was in government, said Burnham.

Defending Lansley, Tory MP Mike Freer said: "The release of the risk register is seen as an opportunity by the opposition to cherry-pick doomsday scenarios the register may contain. It is simply a charter for shroud-waving."

Former shadow health secretary John Healey said: "These current plans are unprecedented in their nature, scale, pace and timing, and that means there is exceptional attention and exceptional concern about the risks associated with their implementation – and that's why there is an exceptional case for releasing this transitional risk register."

Former Labour health secretary Frank Dobson said: "I think the government will finally conclude it's foolish of them not to publish this register because everybody assumes they must have something to hide."