EWaSAP: An early warning sign identification approach based on a systemic hazard analysis

•The safety field still lacks the concepts, tools, and measures to recognise warning signs prior to major failure events.•We propose the Early Warning Sign Analysis based on the STPA (EWaSAP) approach to justify and identify early warning signs.•EWaSAP was used for the identification of early warning signs at a real world drinking water treatment works.•EWaSAP identified early warning signs, which were included in the safety management system at the water treatment works.