A belt for tyres comprising a ply made up of metal cords and two layers of tread ply skim arranged to cover the ply. The ply is coated with a protective composition comprising at least one cross-linkable unsaturated-chain polymer base and a surface-active compound. The protective composition is derived from an aqueous-based suspension applied to said ply. The tread ply skim comprises the same cross-linkable unsaturated-chain polymer base present within the protective composition.