Doctors are failing to detect dementia in hundreds of thousands of patients, a major study has found.
It suggests a third of those affected by the illness do not realise it because they have not been given a formal diagnosis.
The Cambridge-led research, the first of its kind, found even those who were eventually diagnosed had to live with the disease for an average of three years before it was finally detected by GPs.
Many suffer from dementia for more than six years before it is picked up while others die without finding out.
The research will raise serious questions over the Government's dementia strategy. Ministers have pledged to make the UK a world leader in tackling the condition.
A major study earlier this month said dementia was one of the reasons that life expectancy had stalled for the first time in 100 years.
The Alzheimer's Society says early diagnosis is vital. But researchers behind today's study believe some GPs think telling patients and families will only cause anxiety, as there is no cure for dementia.
Another reason is that many sufferers live alone, meaning no one notices if they become more forgetful, confused or start behaving differently.
Others are worried about the stigma of dementia and fear losing friends, their independence and their driving licence.
The research by Cambridge, East Anglia and Newcastle universities, which is still ongoing, hopes to provide one of the most accurate pictures of dementia diagnosis rates to date. It involves 7,796 over-65s in England who were interviewed between 2011 and 2013 for two to three hours. Results were then analysed to see which patients had dementia.
Controversially, the researchers did not tell patients if they tested positively. Instead, they waited to see how much time elapsed before the disease was diagnosed by family doctors.
Earlier this year the researchers contacted the GPs of 458 patients they had identified as having dementia. They found more than a third some 37 per cent had yet to be given a formal diagnosis by the GP. There was not even any mention of symptoms in their notes.
Of the patients who had been diagnosed, there was an average delay of three years between researchers identifying the illness and GPs picking it up.
The researchers calculated a fifth of dementia patients wait six years or more for a diagnosis. Another fifth die before being told they have the disease.
These early results were presented at the world's largest Alzheimer's conference in Docklands, east London, last week.
The full research will be presented at the end of the year.
Dr George Savva, lead author from East Anglia University, said: Many of these people have been living with dementia for six years without diagnosis.
Doctors thought there's no point in diagnosing because there's nothing we can do.
We don't have a lot of evidence as to whether diagnosis helps and the broad consensus is that many people don't really want to know. But there are drugs that can certainly help some people for some time, although not everybody and not much.'
Dr James Pickett, of the Alzheimer's Society, which is funding the study, said: We know people who get a diagnosis can get access to drugs, help and support, they can plan for their future and they can avoid reaching a crisis point.'
East Anglia researcher Clare Aldus said diagnosis helps them understand what is going wrong a diagnosis can bring some relief'.
Separate figures last week suggested approximately one million Britons are living with dementia. This number is expected to double by 2051 due to the ageing population and obesity, which raises the risk.
Ministers have pledged to improve dementia care since Labour issued a National Dementia Strategy in 2009 and former prime minister David Cameron launched the Dementia Challenge in 2012.
GPs were urged to improve diagnosis rates and hospital doctors had training to better detect the illness. In 2014, the NHS paid GPs an extra £55 for every new dementia case they recorded. The scheme was scrapped the next year.
A Department of Health spokesman said it is investing £50million to make hospitals dementia-friendly and £150million to develop a national Dementia Research Institute.
An NHS England spokesman said the number of people with a formal diagnosis has dramatically increased with approximately 150,000 more people able to access early support'.