Housing benefit reforms could leave some elderly people on such low incomes their health may be at risk, the charity Age UK has warned.
It says up to 80,000 pensioners who rent privately face losing an average of around £12 a week in payments.
Age UK fears changes to the way maximum payments are calculated will leave some with less money for food and heating.
But Housing Minister Grant Shapps rejected the claims and said elderly people would not be worse off.
Age UK says it is concerned elderly people will find it harder than others to move home or renegotiate their rent when the reforms take hold.
Policy director Andrew Harrop told the BBC: "We know from the government's own estimates that 80,000 older people are on local housing allowance and all of them would lose out, on average losing £12 a week, some of them losing £30 a week.
"One of our concerns is the government is expecting landlords to drop their rent as a result of this policy and we just don't know if that will happen."
The National Landords Association said it was unrealistic to expect landlords to reduce rents.
Director Richard Price said: "What will probably happen is that there'll be an increasing number of landlords who take LHA tenants at the moment, simply not doing that in the future."
Housing benefit is paid to people on low incomes regardless of whether they work or not.
The coalition government wants to introduce caps ranging from £250 a week for those in a one-bedroom property to £400 for those in a four-bedroom property.
Mr Shapps said there were three reasons why he was confident the reforms would not make elderly people worse off.
He told BBC Radio 4's Today programme: "Number one... there's a very reasonable chance that the landlord may well reduce rents.
"Secondly, there's a very large discretionary fund - 14 times its current size - to help.
"And number three, even under a situation where someone does end up needing to move... the local housing allowance system means there's still up to one third of properties within your local area which you can move to."
The Department for Work and Pensions says it is committed to supporting the most vulnerable and plans an increase in discretionary housing payments.
The government says spending on housing benefit is out of control, costing the government around £21bn a year.
Labour has described the coalition's reforms as "reckless".