This is based on the estimated prevalence levels of people with dementia (756,719) compared to those who have been formally registered as having the disease (430,857).
The report found the Midlands is home to the highest number of undiagnosed people with dementia, with over 60,000 people with the condition not yet diagnosed.
London and the North West have the lowest number of undiagnosed people living with dementia but both regions still have over 30,000 people undiagnosed.
At individual CCG level, Dorset, Norfolk and Fylde and Wyre have the most serious challenges, having some of the highest prevalence rates of dementia but some of the lowest diagnosis rates in the country, the report found.