Dementia sufferers are facing a postcode lottery of diagnosis, a charity has warned.
In some areas of the UK as few as one in three people suffering from the condition will receive a formal diagnosis, while in other areas three quarters of sufferers will be told by a doctor about their condition, according to data released by The Alzheimer's Society.
Across the UK just 46% of sufferers were diagnosed in 2012, the society said.
Health Secretary Jeremy Hunt said the country should be "ashamed" that so many people were being denied treatment which could stave off the condition for years.
He said that doctors are refusing to carry out tests for dementia as they believe it is pointless as there is no effective cure available.
Attitudes in the NHS and in wider society have to change, he added.
"As with cancer in the past, too many health and care professionals are not aware of the symptoms," he wrote in an article for The Telegraph.
"Some even believe that without effective cure there's no point putting people through the anxiety of a memory test - even though drugs can help stave off the condition for several years.
"It is this grim fatalism that we need to shake off. Not just within our health service but across society as a whole.
"It can be a total nightmare getting a diagnosis - and the result is that, shockingly, only 46% of all dementia cases are identified.
"Yet with access to the right drugs and support for a partner, someone can live happily and healthily at home for much longer. We should be ashamed that we deny this to so many people in today's NHS."
The charity said that while the latest figure is an improvement on the previous year, there are still thought to be 428,500 people in the UK who have the condition but have not been diagnosed.
This means they are going without the support, benefits and the medical treatments that can help them live with the condition, charity chief executive Jeremy Hughes said.
Diagnosis rates were best in Scotland where 64.4% of of suffers were told abbot their condition. In Wales, just 38.5% of sufferers formally received a diagnosis in 2012.
And 44.2% of people with the condition in England were diagnosed compared with 63% in Northern Ireland.
Belfast Health and Social Care Trust provided the best diagnosis rates in 2012, with 75.5% of patients being diagnosed.
But in the East Riding of Yorkshire Primary Care Trust, just 31.6% of patients received a formal diagnosis, according to figures produced from data generated by the Government's qualities and outcomes framework.
The charity has produced an interactive map highlighting the variation in dementia diagnosis across the UK.
It said that the new data also suggests that the average waiting time for an appointment at a specialist memory clinic is 32.5 working days - more than the recommended four to six weeks. Some memory clinics reported waiting times of up to 9 months.
Mr Hughes said: "It's disgraceful that more than half of all people with dementia are not receiving a diagnosis, and disappointing to see such a disparity in diagnosis rates in different regions of the UK.
"This goes against best clinical practice and is preventing people with dementia from accessing the support, benefits and the medical treatments that can help them live well with the condition.
"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."
The Alzheimer's Society estimates that 800,000 people in the UK have a form of dementia and more than half have Alzheimer's disease. The figure is estimated to rise to a million by 2021.
Initial signs of the dementia, which is caused by diseases of the brain, may include short-term memory loss that affects every day life, problems with thinking or reasoning, or unexplained anxiety, anger or depression.