Nearly one third of disabled people hit by the so-called bedroom tax have been refused discretionary payments to help them meet the shortfall in their rent.
Demand for the support has tripled this year but applicants face a postcode lottery when it comes to being accepted.
The National Housing Federation found that 29% of disabled residents - more than 3,800 people from the 98 local authorities who responded to Freedom of Information requests - in England who asked for help following the reforms were rejected.
In parts of Kent the success rate was just one in ten, while the level rose to nearly three in North East Derbyshire, Rotherham and parts of Lancashire.
Overall, seven in ten people affected who applied for a discretionary housing payment in the first six months of the policy received one, the study found. But in parts of North Yorkshire the rate fell to just two in ten.
National Housing Federation chief executive David Orr said: "Whenever ministers are challenged on the bedroom tax, they tell us vulnerable people are not at risk because of these discretionary housing payments.
"Now we know the truth: this so-called bedroom tax protection is starting to look like a postcode lottery, with many disabled people and vulnerable families facing miserable odds of getting help.
"Even those who are lucky enough to get support will have to reapply time and time again, each time facing the stress and worry that the funds will be withdrawn, while councils are being inundated with applications.
"This support fund is ineffective and deeply unfair - just like the bedroom tax itself. The only real solution is to repeal it."
The Department for Work and Pensions spokesman insisted the money available to councils to make discretionary payments had gone up significantly.
A spokesman said: "We more than tripled the money we give to local authoritiesto £190m this year to ensure that help was available to those who need it most.
"The cash was given to local authorities to distribute because they deal with their customers on a day-to-day basis and are best placed to see to it that the money reaches those who have the greatest need.
"We will continue to monitor progress closely while our reforms bed down."