Latest figures show diagnosis rates for the condition in England rose just 2 percentage points to 43% in the past year.
This was despite the NHS Operating Frameworks in 2010/11 and 2011/12 prioritising earlier diagnosis.
MPs for the all-part parliamentary group (APPG) on dementia will look into why diagnosis rates remain low, why variation between regions is high, and the advantages of early diagnosis.
GPs have been asked to offer their experiences.
Baroness Sally Greengross, chairwoman of the APPG, said: 'Almost 60% of people with dementia are struggling in the dark without a diagnosis.
'Because their condition has not been recognised they are being denied vital support, information and possible treatments. This can’t carry on.'
Jeremy Hughes, chief executive of Alzheimer’s Society, said: ‘Although diagnosis rates are increasing, the stark reality is that they are not improving fast enough.'
The inquiry has invited GPs and other health and social care workers and commissioners to share their experiences of dementia diagnosis across the UK.