Deputy Lib Dem leader Simon Hughes has fired a warning shot at David Cameron over the PM's idea of ending "council houses for life" in England.
Mr Hughes told BBC News the idea had not been formally discussed by the coalition and Lib Dem MPs would need a lot of persuading to back it.
Mr Cameron said tenants could be given fixed terms of up to 10 years and move on if their circumstances had changed.
Ministers have stressed it would not apply to current tenants.
Housing Minister Grant Shapps said Mr Cameron, who floated the idea at a "PM Direct" public meeting in the West Midlands on Thursday, had been trying to spark debate and any changes - which could also end the right of tenants to pass homes on to their children - would only be considered for future tenancy agreements.
The government has prepared, but not yet published, a consultation document on the idea, which Mr Cameron conceded he expected a row about.
Some local councils have backed the idea of scrapping guaranteed tenancy agreements for life, as they believe it could help ease housing shortages and increase choice for tenants trapped in overcrowded accommodation.
But Simon Hughes told BBC Radio 4's The World at One it would "change the whole nature of public sector housing in England" and urged ministers to proceed with caution.
He said he was not "against radical ideas coming from progressive prime ministers" but he said Mr Cameron had to go through the "proper channels" if he wanted to persuade his coalition partners in the Lib Dems to back the idea.
"It's a prime ministerial idea, it has no more validity yet, and I think our party would need a lot of persuading that it has merit or could work and that's something clearly if he wants us to talk about we're happy to talk about."
He added: "We have to be clear, it is not a Liberal Democrat policy, it is not a coalition policy, it was not in the election manifesto of either party, it was not in the coalition agreement... if he wants to pursue it then there are the proper channels to do so."
Mr Hughes, the MP with the highest number of council tenants as a proportion of the electorate, said Labour had floated a similar idea before withdrawing it "when they saw the pitfalls" and it was important to be sensitive about the way the issue was handled and to consult as widely as possible.
"The fundamental reason why council property is so desirable is because you have security. You know that you can have affordable housing for the rest of your life.
"For people in many walks of life, out of work, retired, on low incomes, that's fundamentally important."
A second Lib Dem MP, Adrian Sanders, has also spoken out against Mr Cameron's idea.
The Torbay MP said: "You don't solve the housing crisis by taking housing away from people who have it to give it to those who haven't got it."
"This isn't something that I can see gaining much support. The coalition agreement is based on two fundamental principles: one - fairness and two - looking after the interests of vulnerable people. So far given what we know about this idea, it fails both those tests."
Mr Cameron's comments were also seized on by Labour, who accused him of threatening the long-term stability people value from secure tenancy.
While Labour MP Bob Ainsworth, responding to Mr Hughes's comments, said: "Proper government can't be conducted if Nick Clegg isn't speaking for the government from the despatch box and David Cameron isn't speaking for the government at a Question and Answer session.
"Clegg and Cameron need to stop giving different answers to different audiences for party political advantage, as has always been the Lib Dem tactic."