Step 1: Ask which factor most often drives the long-standing, fibrosing pancreatic damage that defines chronic pancreatitis.
Step 2: Across populations, sustained heavy alcohol use is the dominant aetiology, accounting for as much as 70 percent of cases.
Step 3: Intraductal calculi tend to form as a result of the chronic process, and pancreas divisum is only an occasional anatomical contributor.
Step 4: Gallstones are tied to acute pancreatitis rather than the chronic form, so the leading cause here is chronic alcohol.
\[\boxed{\text{Chronic alcohol}}\]