Purpose: To assess the incidence of undertriage in major trauma, its determinant, and association with mortality.Methods: A multicentre retrospective cohort study was conducted using data from a French regional trauma registry (2011-2017). All major trauma (Injury Severity Score ≥ 16) cases aged ≥ 18 years and managed by a physician-led mobile medical team were included. Those transported to a level-II/III trauma centre were considered as undertriaged. Multivariable logistic regression was used to identify factors associated with undertriage.Results: A total of 7110 trauma patients were screened; 2591 had an ISS ≥ 16 and 320 (12.4%) of these were undertriaged. Older patients had higher risk for undertriage (51-65 years: OR = 1.60, 95% CI [1.11; 2.26], p = 0.01). Conversely, injury mechanism (fall from height: 0.62 [0.45; 0.86], p = 0.01; gunshot/stab injuries: 0.45 [0.22; 0.90], p = 0.02), on-scene time (> 60 min: 0.62 [0.40; 0.95], p = 0.03), prehospital endotracheal intubation (0.53 [0.39; 0.71], p < 0.001), and prehospital focussed assessment with sonography [FAST] (0.15 [0.08; 0.29], p < 0.001) were associated with a lower risk for undertriage. After adjusting for severity, undertriage was not associated with a higher risk of mortality (1.22 [0.80; 1.89], p = 0.36).Conclusions: In our physician-led prehospital EMS system, undertriage was higher than recommended. Advanced aged was identified as a risk factor highlighting the urgent need for tailored triage protocol in this population. Conversely, the potential benefit of prehospital FAST on triage performance should be furthered explored as it may reduce undertriage. Fall from height and penetrating trauma were associated with a lower risk for undertriage suggesting that healthcare providers should remain vigilant of the potential seriousness of trauma associated with low-energy mechanisms.