A method of manufacturing a portion of a wind turbine blade, especially a blade shell half, said wind turbine blade portion comprising an aerodynamic blade shell member and an integrated longitudinally extending spar cap or main laminate in a predetermined longitudinal area of the shell member, said area having a length and a width defined by a first and a second lateral side (46, 48), the method comprising the step of: a. providing a preformed at least substantially cured aerodynamic shell member of a primary resin reinforced by a primary fibre material and having an outer surface and an inner surface (54, 52) b. arranging a lower resin flow medium on the inner surface of the shell member in a longitudinal portion thereof corresponding at least to the predetermined longitudinal area and extending beyond the first and the second lateral side (46, 48) of the predetermined area c. laying up layers of a secondary fibre material comprising carbon fibre for the spar cap or main laminate on the lower resin flow medium in a longitudinal portion thereof corresponding to the predetermined area d. arranging an upper resin flow medium on the laid-up secondary fibre material so as to extend laterally beyond the first lateral side of the predetermined area and overlap the lower flow medium in a longitudinal overlap zone at the first lateral side e. arranging a first longitudinal resin inlet channel on the upper and/or the lower resin flow medium (70, 64) laterally outwardly of the first lateral side, and preferably on the upper flow medium in the longitudinal overlap zone at the first lateral side of the predetermined area f. arranging a first longitudinal vacuum channel on/or above the inner surface of the preformed shell member laterally outwardly of the second lateral side, and preferably on the lower flow medium outwardly of the second lateral side of the predetermined area g. sealingly arranging a vacuum bag over at least the secondary fibre lay-up, the vacuum channel, the resin inlet channel and the lower and upper resin flow medium (70, 64) to define a mould cavity h. applying a negative pressure to the mould cavity through the vacuum channel i. supplying a secondary resin to the fibre-reinforcing material in the mould cavity through the resin inlet channel j. allowing the resin to cure to form an integrated fibre reinforced spar cap or main laminate on the blade shell member, wherein the inner surface of the preformed aerodynamic shell member is provided with a longitudinal recess corresponding at least essentially to the predetermined longitudinal area and at least portions of the secondary fibre material arranged therein in step d. a. providing a preformed at least substantially cured aerodynamic shell member of a primary resin reinforced by a primary fibre material and having an outer surface and an inner surface (54, 52) b. arranging a lower resin flow medium on the inner surface of the shell member in a longitudinal portion thereof corresponding at least to the predetermined longitudinal area and extending beyond the first and the second lateral side (46, 48) of the predetermined area c. laying up layers of a secondary fibre material comprising carbon fibre for the spar cap or main laminate on the lower resin flow medium in a longitudinal portion thereof corresponding to the predetermined area d. arranging an upper resin flow medium on the laid-up secondary fibre material so as to extend laterally beyond the first lateral side of the predetermined area and overlap the lower flow medium in a longitudinal overlap zone at the first lateral side e. arranging a first longitudinal resin inlet channel on the upper and/or the lower resin flow medium (70, 64) laterally outwardly of the first lateral side, and preferably on the upper flow medium in the longitudinal overlap zone at the first lateral side of the predetermined area f. arranging a first longitudinal vacuum channel on/or above the inner surface of the preformed shell member laterally outwardly of the second lateral side, and preferably on the lower flow medium outwardly of the second lateral side of the predetermined area g. sealingly arranging a vacuum bag over at least the secondary fibre lay-up, the vacuum channel, the resin inlet channel and the lower and upper resin flow medium (70, 64) to define a mould cavity h. applying a negative pressure to the mould cavity through the vacuum channel i. supplying a secondary resin to the fibre-reinforcing material in the mould cavity through the resin inlet channel j. allowing the resin to cure to form an integrated fibre reinforced spar cap or main laminate on the blade shell member,