1. An extension tube for a urinary catheter, comprising a tubular body with an inlet opening, an outlet opening and an internal lumen extending between said inlet opening and said outlet opening, wherein the tubular body has a length in the range of 30-70 cm and, in an erected state, a maximum cross-sectional extension in the range of 0.5-3 cm, said tubular body comprising at least a first layer, said first layer including natural and/or regenerated cellulose fibers, and wherein the tubular body is arranged to be compressible into a generally flat compacted storage state, and to be expandable into an expanded, tubular use state, wherein the tubular body has a plurality of longitudinally extending fold lines, extending along an axial direction of the tubular body, wherein the tubular body is arranged to remain intact and stable during a flow of water or urine through the internal lumen for at least 5 minutes.