Research from the Alzheimer's Society predicts more than a million people will have dementia by the start of the next decade, but unless detection rates improve, more than half a million people will be living with undiagnosed dementia by 2021, the study found.
The Alzheimer's Society said earlier diagnosis could reduce NHS costs as hospital stays were shorter. Support and care was improved.
The charity has developed an interactive map based on the findings of its research. GPs can use it to identify current and future rates of dementia diagnosis in each PCT and SHA or health board across the UK.
Dorset has the lowest rate of diagnosis with just 25% of people with dementia receiving a confirmed diagnosis.
People in Belfast had the highest rate at 69%.
Jeremy Hughes, Alzheimer's Society chief executive, said: 'It's really concerning that 60% of people with dementia don't officially know they have the disease and can't access the support they so badly need.'
He added: 'Anyone who is concerned about their memory should visit their GP.'
The charity said it would seek to increase awareness and understanding of dementia among GPs as 'a major priority'.
It added that a formal diagnosis is vital for people with dementia, as without it they cannot access drugs or advice on how to manage their condition.