Welsh hospitals continue to miss A&E targets of seeing 95% of patients within four hours of arrival.
Dr Paul Myres will highlight the significant level of demand for non-serious or life-threatening conditions when addressing AMs on Wednesday.
Between 15-26.5% of cases could have been dealt with by GPs, he will say.
It comes at a time when a senior nurse at Wales' biggest hospital - University Hospital of Wales, Cardiff - says that the stress A&E staff are under is worse than she faced on the front line during the second Iraq war.
Dr Myres is using research by Deloitte to address AMs he has been invited to speak to.
"Waiting times are increasing year on year and we know that the reason a lot of people attend A&E unnecessarily is because they cannot access a GP in time," he will say.
"What this research shows is that a significant percentage of the cases presenting at A&E could be dealt with in general practice.
"Unless we invest substantially in expanding the GP workforce in Wales, general practice is at risk of going into meltdown - with the profession's ability to deliver excellent patient care increasingly compromised."
An increase in access to GPs would lead to a drop in the number of people going to A&E, the chairman of the Royal College of GPs will tell AMs.