Apparatus for forming a collar of a double packet, wherein said collar comprises at least a base sheet on which at least a central rib and at least two lateral flaps protruding from said base sheet are made, said forming apparatus being characterized in that it comprises a forming device rotatable intermittently and provided with one or more heads to hold the base sheet of the collar in position in an extended condition, in order to move said base sheet along at least one segment of a circular operating path between a supply station, a folding station and a transfer station, wherein the intermittent rotation of said forming device is coordinated at least with the angular position of said supply station, folding station and transfer station along said operating path, said forming device being configured to cooperate selectively and by means of said intermittent rotation along said operating path with: a supply device to supply said base sheet, disposed at said supply station (P 1) and provided with at least a support element to support the base sheet translatable with respect to said forming device; a folding device to fold said base sheet, disposed at said folding station and provided with at least a folding element translatable in a folding direction with respect to the base sheet and by means of which at least a partial folding of the base sheet is performed, such that said central rib is created and said collar assumes a T-shaped conformation; and a mobile transfer device, disposed at said transfer station and configured to transfer the at least partly folded collar from said head of the forming device to a wrapping device, intermittently rotatable in a manner coordinated with the intermittent rotation of said forming device and provided with one or more pockets to complete the folding of the collar, wherein said one or more pockets of the wrapping device comprise at least one pair of opposite lateral walls to complete the forming or folding of the collar, said lateral walls being configured to form said lateral flaps of the collar, which therefore assumes a final double U-shape inside the pocket during the insertion of the T-shaped collar into the pocket itself. a supply device to supply said base sheet, disposed at said supply station (P 1) and provided with at least a support element to support the base sheet translatable with respect to said forming device; a folding device to fold said base sheet, disposed at said folding station and provided with at least a folding element translatable in a folding direction with respect to the base sheet and by means of which at least a partial folding of the base sheet is performed, such that said central rib is created and said collar assumes a T-shaped conformation; and a mobile transfer device, disposed at said transfer station and configured to transfer the at least partly folded collar from said head of the forming device to a wrapping device, intermittently rotatable in a manner coordinated with the intermittent rotation of said forming device and provided with one or more pockets to complete the folding of the collar, wherein said one or more pockets of the wrapping device comprise at least one pair of opposite lateral walls to complete the forming or folding of the collar, said lateral walls being configured to form said lateral flaps of the collar, which therefore assumes a final double U-shape inside the pocket during the insertion of the T-shaped collar into the pocket itself.