A method of manufacturing an elongated composite structure having two separate longitudinal composite structure sections, where said method comprises the following steps: providing a rigid mould part 13 having a first forming surface 14 with a contour defining an outer surface of the elongated composite structure, arranging a first fibre lay-up 15 in a first longitudinal section 16 of the first mould part 13, arranging a first flexible foil 18 over a first crosswise edge area 17 of the first fibre lay-up 15, arranging a second fibre lay-up 20 in a second longitudinal section 21 of the mould part 13 so as to overlap the crosswise edge area 17 of the first lay-up 15 and thereby the first flexible foil 18 in an overlap area 22 forming an interface between the fibre lay-ups 15,20, providing polymer to the fibre lay-ups 15,20 and allowing the polymer to cure.