Inmates clashed at Negombo Prison and left 25 people dead with more than 100 others hurt.
The fighting broke out Sunday and continued into Monday.
Rival groups of prisoners first argued over drug issues during breakfast.
Then they attacked guards, seized weapons, and tried to rush the gate.
Security forces fired shots to regain control.
Prison officials say the unrest started when one group targeted another over a drug informant.
Chamika Gajanayake, the prisons department spokesperson, confirmed the clashes involve drug trafficking disputes.
Hospital staff at Negombo treated the injured on site.
Doctors moved 18 people with serious wounds, including gunshot injuries, to a larger facility in Colombo.
Workers placed the bodies at the hospital morgue.
This event highlights long-standing problems in the prison system.
Facilities hold far more people than they were built for.
Authorities now plan transfers and continue their investigation.
Police have not released further details as the probe moves forward.
