Lib Dem deputy leader Simon Hughes is threatening a backbench rebellion over planned cuts to housing benefit.
The party's deputy leader told Channel 4 News some of the proposals were "harsh and draconian".
In its Spending Review last week, the government announced major changes to housing benefit - including cutting it by 10% for the long-term jobless.
Labour has offered to join forces with Lib Dem backbenchers to force the government to rethink the policy.
'Loud and clear'
The government is proposing the biggest shake-up in housing in decades - cutting money for new social housing by 50% and allowing housing associations to charge new tenants close to the full market rate for rent.
The government hopes the changes will lead to more social housing being built - but critics fear an exodus of poor people from the inner cities as they are forced out by higher rents and a new cap on housing benefit payments.
Single people under 35 will have to live in shared accommodation if they are claiming housing benefit. The long-term unemployed also face tougher sanctions.
Mr Hughes, whose Bermondsey and Old Southwark constituency has the most social housing of any in the country, said he was particularly opposed to the plans to cut housing benefit from 2013 by 10% for those who had been on Jobseekers allowance for 12 months.
"My message to the government is, I don't think you will get Parliamentary approval for your current plans," he said.
"I think the government understands there has to be negotiations.
"The current proposals are not the best set of proposals, whatever the financial constraints. There are better ways of doing it and we need to achieve them.
"I am making sure the message from me and many colleagues is being communicated loud and clear to government."
Mr Hughes said he believed the Spending Review was fair "as a whole".
"I believe it is far fairer because Lib Dems are there than if it had been a Tory-only budget," he said. "I believe it is broadly fair in that the rich will pay most and most of the poor will be protected."
Mr Hughes' backbench Lib Dem colleague Tom Brake said he could not support the proposals on housing benefit either and called for a government rethink.
"My concern is that the changes on housing benefit, particularly in London, are going to have a really hard-hitting impact on a number of families," the Carshalton and Wallington MP told BBC News.
He said he agreed with Simon Hughes that the Spending Review overall was "fair" - but he said London MPs were concerned the housing benefit changes, particularly a £400 cap on claims, will force poor families to move out of the city.
He said the coalition would not get the proposals through as they stood and said negotiation was needed with ministers to "smooth off some of the rough edges and make sure this package is truly fair and doesn't have a disproportionate effect on families in London".
On Sunday, Nick Clegg, the Lib Dem leader and deputy prime minister, defended planned cuts to housing benefit, saying it was not fair that people who went out to work got less help with accommodation than those who did not.
Mr Clegg told the BBC's Andrew Marr Show the government's plans would create more social housing and were "fair" on housing benefit claimants.
He said: "We need to do something about a housing benefit bill which has gone up from £10bn to £21bn in recent years under Labour and there haven't been enough affordable homes built."
Business Secretary Vince Cable, a fellow Lib Dem MP who also represents a London seat, Twickenham, also said the key issue was not housing benefit - but building more social housing.
"Simon cares passionately about social housing and that reflects his many years as an MP in Bermondsey and he shares my frustration, I'm also a London MP, about the way over many years there simply hasn't been enough social housing to meet demand and that's the issue we have to deal with rather than the intricacies of the housing benefit regulation.
"We have to increase the supply of social housing. It's absolutely crucial."
But shadow work and pension secretary Douglas Alexander, for Labour, said Mr Hughes' comments showed "even the Liberal Democrat deputy leader doesn't believe the government's housing benefit cuts are fair".
He said the proposals were not fair on housing benefit claimants who were genuinely seeking work.
"We should be working to guarantee jobs for the long-term unemployed, not risking homelessness for those who are doing their best to find work," he said.
"I now urge Simon Hughes to back up these words and, with us and other Lib Dem MPs, to force the government to think again."