The present invention relates to a sheath (7) for a cable (1), the sheath comprising, at least in part, a halogen-free intumescent composition, said sheath being characterized in that said composition contains a mixture of: a polymer matrix chosen from a polyethylene, a polypropylene, an ethylene copolymer, a propylene copolymer, a silicone, a polyamide, and a mixture thereof; and at least one flame-retardant additive constituted by a first inorganic compound having a lamellar crystal structure, and by a second inorganic compound inserted between the lamellae of said first compound, said second compound being capable of causing said lamellae to move apart under the effect of heat. This sheath is for use as a flame retardant in telecommunications cables.