As many as 100,000 people qualify for a substantial council tax discount but are not getting it.
That's according to a new report from MoneySavingExpert.com that shows anyone living with someone diagnosed as 'Severely Mentally Impaired' - with conditions like dementia and others - are eligible for certain benefits.
See also: Sharp rise in council tax rebate scams
These include a 25% council tax discount, while those living alone are entitled to a 100% discount.
"The law rightfully recognises that many vulnerable people who have a severe mental impairment such as dementia or Alzheimer's are entitled to a council tax reduction," said MoneySavingExpert.com founder of Martin Lewis.
"Sadly many councils do not and the result is that society's most vulnerable face a postcode lottery with huge variance in take up depending on where people live.
"I would urge any family affected by this to speak to their local council. Not only are many due a reduction of £400+ a year, but it can be backdated for years. We've already had a number of successes where people have been paid back £1,000s – though this again depends on a postcode lottery."
The scale of the problem
After feedback form readers, MoneySavingExpert submitted Freedom of Information Act requests to 265 councils across the country and checked up on 100 councils itself.
The investigation revealed a shocking difference in the number of people claiming the discount.
For example uptake in Renfrewshire is 77 times higher than just 40 miles away in East Ayrshire.
Worse, more than two in three of the councils investigated by the site provided incorrect information about the discount.
Council tax staff were unable to clearly explain the criteria for eligibility or how to submit a claim for the discount.
"Local councils need to urgently review their processes in order to reduce the amount of misinformation given out by frontline staff," Lewis said.
"The people who are missing out on this are amongst the most vulnerable in our society and can't fight for themselves. They should not be left to battle this alone."