The government says it has not decided whether to appeal against a tribunal ruling to make public a risk assessment of the NHS shake-up in England.
Minister Lord Howe said the transition risk register would not be published until the Freedom of Information tribunal had explained its ruling.
But Labour peers said Parliament could not properly scrutinise the health bill without seeing the register.
The bill is in the final stages of its passage through Parliament.
The Department of Health had appealed against an FOI ruling that the transition risk register, requested by Labour MP John Healey, be published.
But it lost last week, despite civil servants' warnings that to publish confidential advice could have a "chilling effect".
The government had used the "section 35" defence under the Freedom of Information Act, which exempts information used in policy formulation and development from having to be released.
But it must be weighed against the balance of public interest - and in an earlier ruling the information commissioner had said in this case, that was "very strong".
Lord Howe told the House of Lords on Monday the tribunal had upheld the government's position that the DoH should not publish the strategic risk register, but ordered that it should release the transitional register.
"However, we await the full judgement which will contain the detailed reasoning for the decision," he said.
"This makes it extremely difficult to make a decision on whether or not the government wishes to appeal this decision.
"I hope very much the tribunal will give its full judgement as soon as possible."
But Baroness Royall, Labour leader in the Lords, said Parliament had to make "critical decisions" about the future of the NHS in the next 10 days and without the information contained in the register, it would be "less well-informed" than it otherwise would be.
"Why is the government therefore preventing Parliament from having the best possible information on the NHS so it can make the best possible decisions about the NHS?" she asked.
In reply, Lord Howe said many of the concerns in the register had already been "extensively aired".
He argued there was "a matter of principle" at stake and "no government" had routinely made risk registers available.
Labour's Lord Harris said dispute was not over routine releases, but one "exceptional" case concerning a particularly controversial bill.
But Lord Howe said civil servants, who compile the risk register, wished to have "safe space in which to think the unthinkable" and releasing it would take that away.