Six Welsh charities have urged the UK Government to rethink proposed cuts to housing benefit.
They said the measures were "all stick and no carrot" and that they would hit the most vulnerable hardest.
Chancellor George Osborne last week announced plans to cap the combined income from benefits.
The Department for Work and Pensions (DWP) said housing benefit had "spiralled out of control".
The changes are being brought in partly to tackle perceived long-term welfare dependency by unemployed people, with some saying they are better off on benefits than working.
Shelter Cymru, Mind Cymru, Citizens' Advice Cymru, Gofal, Community Housing Cymru and Cymorth have joined forces with the Chartered Institute of Housing Cymru, the National Landlords Association and the Welsh Tenants Federation.
Their joint statement says the proposed changes "will have a devastating impact on the most vulnerable people in our society".
It adds: "We urgently call on the government to reconsider the cuts, which will disproportionately affect those in greatest need, including homeless and vulnerable families, those in need of housing-related support, people with mental health problems and children living in poverty, and result in greater long-term economic and social costs for local authorities, housing associations and government in Wales. "
John Puzey, director of Shelter Cymru, said they appreciated the need for reform and that the current sytemdisincentivised people from work.
He added: "However, it seems that these proposals are all stick and no carrot and we are all extremely concerned about the long-term implications for families, individuals and communities in Wales."
The government has not spelled out exactly how benefits will change.
But proposals include restricting housing allowance to homes with a maximum of four bedrooms and cutting housing benefit to cover only the lowest 30% of rents in an area, instead of the average.
Another proposal is to cut housing benefit by 10% for anyone who has been on Job Seekers Allowance for more than 12 months, regardless of efforts to find work.
The charities believe cutting the local housing allowance will leave many facing the threat of homelessness.
Fran Targett, director of Citizens Advice Cymru, said: "A report by the DWP shows that almost 100% of LHA (Local Housing Allowance) recipients in Wales will be worse off by an average of £9 a week.
"Many people who receive LHA already make up a shortfall between their benefit and the cost of their rent and when you're on a very tight budget, just a few pounds a week can make the difference between coping or not."
A spokesperson for the DWP said: "Housing benefit has spiralled out of control over the last decade and we have been left with a system that plunges thousands of vulnerable people in an unbreakable cycle of benefit dependency and costs the taxpayer billions every year.
"Our reforms mean that if you need a roof over your head, we will provide it, but they also put people on benefits on an equal footing with working families, rather than in accommodation they could never afford to maintain upon entering into work."