An electrical cable for very high voltage underground power transmission comprises two or more inner conductors (2) which are twisted together and are supported within the gas-filled void of a hollow extruded plastic cable sheath (4) by means of a plurality of spacers (3) which are positioned along the length of the cable, each spacer (3) being formed of solid dielectric material with a number of angularly spaced radial webs equal to the number of conductors in the cable and being engaged with the twisted conductors with the webs extending from the cable axis radially outwardly between the conductors into contact with the inner wall of the cable sheath (4) where the outermost ends of the webs engage with complementarily shaped portions (5) of the sheath (4) providing for reduction of the electrical stresses at the spacer ends. The cable is formed by extrusion of the sheath over the twisted core conductors after insertion of the spacers, and is vacuum corrugated after extrusion, the corrugator being specially formed and operated so as to accommodate the locations whereat the spacer ends engage with and locally deform the extruded sheath. The plastic sheath can be formed with multiple layers by a multiple extrusion process, and a metal outer sheath may be formed integrally with the cable and may have corrugations which mate closely with the corrugations of the plastic sheath.