Almost two thirds of people living with Alzheimer’s in Sussex have not been diagnosed.
There are more than 24,400 residents across the county believed to have the condition, but 60 per cent of them have never been told.
It means thousands of people are not getting access to the vital treatment they need to help control the progress of their illness.
New figures published by the Alzheimer’s Society reveal the number of people with dementia is expected to rise throughout Sussex to more than 33,700 over the next eight years.
This is partly because of the large number of elderly people in the county, and is expected to place more pressure on NHS and council services.
Officials estimate there are 3,098 people with Alzheimer’s in Brighton and Hove alone, but just 37 per cent of them have been diagnosed.
The number of cases is expected to rise to 3,340 by 2021.
East Sussex Downs and Weald has 6,736 people with the condition and a diagnosis rate of 38 per cent while Hastings and Rother has 3,485 with 44 per cent diagnosed.
More can be done In West Sussex, there are 13,887 people with dementia and 39 per cent have been diagnosed.
Jeremy Hughes, from the Alzheimer’s Society, said: “It’s disgraceful that more than half of all people with dementia are not receiving a diagnosis.
“Studies show that an early diagnosis can save the taxpayer thousands of pounds, because it can delay someone needing care outside of their own home.
“The NHS has already made a commitment to improving diagnosis rates but more needs to be done to ensure people with dementia are able to live as well as possible with the disease.”
A spokesman for NHS Sussex said: “We are working closely with health and social care professionals to develop the services available to support early diagnosis and ongoing care and treatment for people with dementia and their families and carers.
“As part of this, memory assessment services are being introduced across Sussex, and these will help to increase the number of people who receive a formal dementia diagnosis.
"Significant funding has also been awarded to specific projects in the county from the Prime Minister’s Dementia Challenge Fund to improve the services and support available for people with dementia and their carers.”