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

Inclusion Criteria:

• Diagnosed with type 1 diabetes mellitus greater than or equal to (≥) 1 year beforescreening.

• Treated with multiple daily insulin injections (daily basal insulin analogue andbolus insulin analogue regimen) ≥ 6 months before screening.

• HbA1c from 7.0-10.0 percentage (%) (53.0-85.8 millimoles per mole (mmol/mol)), bothinclusive, at screening confirmed by central laboratory analysis.

• Ability and willingness to adhere to the protocol including performance ofself-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 informedconsent.

• Female who is pregnant, breast-feeding or intends to become pregnant or is ofchildbearing potential and not using adequate contraceptive method.

• Exposure to an investigational medicinal product within 90 days or 5 half-lives ofthe investigational medicinal product (if known), whichever is longer, beforescreening.

• Any condition, except for conditions associated with type 1 diabetes mellitus, whichin the investigator's opinion might jeopardise participant's safety or compliancewith the protocol.

• Anticipated initiation or anticipated change in concomitant medications (for morethan 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 toClarke's questionnaire question.

• Recurrent severe hypoglycaemic episodes within the last year as judged by theinvestigator.