A Research Study to See How a Weekly Insulin, Insulin Icodec, Helps in Reducing the Blood Sugar Compared to Daily Insulin Glargine, Both in Combination With Insulin Aspart, in Adults With Type 1 Diabetes

This study compares insulin icodec, an insulin taken once a week to insulin glargine, an insulin taken once a day. The study medicine will be investigated in participants with type 1 diabetes. The study will look at how well insulin icodec taken weekly controls blood sugar compared to insulin glargine taken daily. The study will last for about 8.5 months.

Inclusion Criteria: * Diagnosed with type 1 diabetes mellitus greater than or equal to (≥) 1 year before screening. * Treated with multiple daily insulin injections (daily basal insulin analogue and bolus insulin analogue regimen) ≥ 6 months before screening. * HbA1c from 7.0-10.0 percentage (%) (53.0-85.8 millimoles per mole (mmol/mol)), both inclusive, at screening confirmed by central laboratory analysis. * Ability and willingness to adhere to the protocol including performance of self-measured plasma glucose (SMPG) profiles, based on the investigator's judgement. Exclusion Criteria: * Known or suspected hypersensitivity to study intervention(s) or related products. * Previous participation in this study. Participation is defined as signed informed consent. * Female who is pregnant, breast-feeding or intends to become pregnant or is of childbearing potential and not using adequate contraceptive method. * Exposure to an investigational medicinal product within 90 days or 5 half-lives of the investigational medicinal product (if known), whichever is longer, before screening. * Any condition, except for conditions associated with type 1 diabetes mellitus, which in the investigator's opinion might jeopardise participant's safety or compliance with the protocol. * Anticipated initiation or anticipated change in concomitant medications (for more than 15 consecutive days) known to affect weight or glucose metabolism (e.g., treatment with thyroid hormones, or systemic corticosteroids). * Known hypoglycaemic unawareness as indicated by the Investigator according to Clarke's questionnaire question. * Recurrent severe hypoglycaemic episodes within the last year as judged by the investigator.