The present invention relates to an insulating composition for an electric power cable which comprises a polyolefin, an antioxidant, and a polar copolymer. Further, the present invention relates to an electric power cable comprising an insulating layer including a composition according to the present invention, and to the use of a polar copolymer for improving the storage stability, i.e. reducing the exudation of an antioxidant, in an insulating polymer composition. Thereby, said composition comprises polar monomer units in a comparatively small amount, e.g. in an amount of polar monomer units in the total polymer part of the composition from 1 to 100 micromol (1·10″6 to 100·106 mol) per gram of polymer in addition to an antioxidant.