BACKGROUND AND AIMS: The simplified magnetic resonance enterography (MRE) index (sMARIA), London and "extended" London scoring systems are widely used in Crohn's disease (CD) to assess disease activity, although validation studies have usually been single centre, retrospective and/or used few readers. Here, we evaluated these MRE indices within a prospective multicentre, multireader diagnostic accuracy trial. METHODS: A subset of participants (newly diagnosed or suspected of relapse) recruited to the METRIC trial with available terminal ileal (TI) biopsies was included. Using pre-specified thresholds, the sensitivity and specificity of sMARIA, London and "extended" London scores for active and severe (sMARIA) TI CD were calculated using different thresholds for the histological activity index (HAI). RESULTS: We studied 111 patients (median 29 years, interquartile range 21-41, 75 newly diagnosed, 36 suspected relapse) from 7 centres, of whom 22 had no active TI CD (HAI=0), 39 mild (HAI=1), 13 moderate (HAI=2), and 37 severe CD activity (HAI=3). In total, 26 radiologists prospectively scored MRE datasets as per their usual clinical practice. Sensitivity and specificity for active disease (HAI>0) were 83% (95% confidence interval 74-90%) and 41% (23-61%) for sMARIA, 76% (67-84%) and 64% (43-80%) for the London score, and 81% (72-88%) and 41% (23-61%) for the "extended" London score, respectively. The sMARIA had 84% (69-92%) sensitivity and 53% (41-64%) specificity for severe CD. CONCLUSIONS: When tested at their proposed cut-offs in a real-world setting, sMARIA, London and "extended" London indices achieve high sensitivity for active TI disease against a histological reference standard, but specificity is low.