A method of manufacturing an elongated composite structure having a longitudinal axis and comprising at least a first and a second separate longitudinal composite structure section (16, 21) and being formed of reinforced polymer material including a polymer matrix and fibre material embedded in the polymer matrix, wherein the method comprises the followings steps: a) providing a first rigid mould part having a first forming surface with a contour defining an outer surface of the elongated composite structure, b) arranging a first fibre lay-up in a first longitudinal section of the first mould part, the first fibre lay-up defining a first crosswise edge area, c) arranging at least one first flexible and preferably resilient foil, preferably a polymer foil, over at least the first crosswise edge area of the first fibre lay-up, d) arranging a second fibre lay-up in a second longitudinal section of the first mould part so that the second fibre lay-up overlaps the first crosswise edge area of the first lay-up and thereby the first flexible foil in an overlap area forming an interface between the first fibre lay-up and second fibre lay-up, e) providing polymer to the first and second longitudinal sections simultaneously with steps b) and d), respectively, and/or after step d), and f) curing or allowing the polymer to cure so as to form the first separate longitudinal composite structure section and the second separate longitudinal composite structure section, the method being characterized in g) separating the first separate longitudinal composite structure section and the second separate longitudinal composite structure from each other at the overlap area forming an interface, and H) later assembling the first and second longitudinal composite structure sections (16, 21) at the interface so as to form the elongated composite structure. a) providing a first rigid mould part having a first forming surface with a contour defining an outer surface of the elongated composite structure, b) arranging a first fibre lay-up in a first longitudinal section of the first mould part, the first fibre lay-up defining a first crosswise edge area, c) arranging at least one first flexible and preferably resilient foil, preferably a polymer foil, over at least the first crosswise edge area of the first fibre lay-up, d) arranging a second fibre lay-up in a second longitudinal section of the first mould part so that the second fibre lay-up overlaps the first crosswise edge area of the first lay-up and thereby the first flexible foil in an overlap area forming an interface between the first fibre lay-up and second fibre lay-up, e) providing polymer to the first and second longitudinal sections simultaneously with steps b) and d), respectively, and/or after step d), and f) curing or allowing the polymer to cure so as to form the first separate longitudinal composite structure section and the second separate longitudinal composite structure section, the method being characterized in g) separating the first separate longitudinal composite structure section and the second separate longitudinal composite structure from each other at the overlap area forming an interface, and H) later assembling the first and second longitudinal composite structure sections (16, 21) at the interface so as to form the elongated composite structure.