The present disclosure provides a method of manufacturing a composite sandwich panel having a core formed of a plurality of cells extending vertically between a first skin and a second skin. The method includes creating at least one first strip and a second strip from a temporary material, each strip having at least one cavity having a succession of aligned half-cells, lining the cavity of the first strip with a fibrous ply, assembling the first strip and the second strip by interlocking the cavity of the first strip with the cavity of the second strip and trapping the fibrous ply therebetween, and trimming excess temporary material of the entirety of the strips formed during the preceding assembly step so as to form a new cavity which forms a succession of aligned half-cells.