Primary Objective: To demonstrate that morning injection of Toujeo (HOE901-U300) compared to Lantus provides better glycemic control evaluated by Continuous Glucose Monitoring (CGM) in adult participants with type 1 diabetes mellitus. Secondary Objective: To demonstrate that treatment with HOE901-U300 compared to Lantus provides: * Lower incidence rate of nocturnal symptomatic hypoglycemia; * Better glucose control coverage during the last hours of CGM before next basal-insulin dosing; * Less variability in CGM profile.

The maximum study duration per participant was to be of approximately 20 weeks that consisted of an up to a 4-week screening and CGM training period including a 1-2 week baseline (blinded) CGM performance (allowed for re-training), a 14-week open-label, comparative treatment period allowing for dose titration in both basal and meal-time insulin and including a 1-2 week end-of treatment blinded CGM collection with fixed dose of HOE901-U300 and Lantus, and a 2 day post treatment follow-up period.