Electing the House of Lords without clearly defining its powers could lead to its "wanton destruction", Baroness Boothroyd has warned.
The former Commons Speaker said she would offer "unremitting opposition" to reform plans until they were clarified.
Ministers are due to publish draft legislation providing for an elected second chamber by the end of this year.
Deputy Prime Minister Nick Clegg has said years of talking about reform must come to an end.
A cross-party committee on House of Lords reform - tasked with producing a draft bill setting out proposals for a wholly or mainly elected second chamber - began work in June.
Speaking then, Mr Clegg said giving the chamber a democratic mandate was a key part of what he said would be the most ambitious constitutional reform package since the Great Reform Act of 1832.
'New political class'
Mr Clegg is likely to be pressed on the details of the bill when he appears before the Lords Constitution Committee on Wednesday.
But, while the Lib Dems are largely supportive of a fully elected House of Lords, many Conservatives are lukewarm and Prime Minister David Cameron has suggested the issue is not an immediate priority for the government.
In a debate, Baroness Boothroyd told peers they must be "vigilant" over the government's approach as it had said nothing about the principles underpinning its thinking and its view of the Lords' powers vis-a-vis the Commons.
If ministers wanted a "serious dialogue" over Lords reform, they had to do more than "pay lip service" to openness.
"Vague assurances on a vital issue of constitutional reform will simply not do," Baroness Boothroyd, a former Labour MP, said.
"Until we know precisely what powers a reformed second chamber will have, we cannot possibly subscribe to the wanton destruction of this house in the interests of a new political class that lacks acknowledged expertise and cherished independence of this institution."
Conservative peer Lord Forsyth said he believed in the constitutional supremacy of the House of Commons and would not support anything that threatened to undermine that.
Electing the Lords, he suggested, could also lead to a drain of expertise as he believed that very few existing members would be willing to stand for election and they were likely to be succeeded by a "B-team".
"This chamber works," he told peers. "Leave it alone. It is not broken."
But former Labour minister Lord Adonis said the House needed to focus instead on "improving" its current procedures.
During his time as transport secretary in the last government, he said he had never been called to give evidence to a Lords committee and peers had not debated the major issues in his portfolio such as Heathrow expansion and high-speed rail.
"A century and a half ago Walter Bagehot said that the cure for admiring the House of Lords was to come and look at it," he said. "I fear that this is still too often true today."
"Being objective about ourselves, we are diligent and public spirited, now and then we strike a chord on issues of public moment and occasionally we act as a constitutional backstop.
"But we have failed to develop modern procedures or committees for scrutiny and across large swathes of public policy we are practically non-existent as a parliamentary assembly."
For the government, Lord Taylor of Holbeach said the cross-party reform committee would examine the functions, powers and conventions of a reformed House as part of its deliberation over future options.
Once the bill was published, he said there would be plenty of time for "constructive debate" and scrutiny would not be rushed.