A process for preparing, 1,1,3-trioxo-1,2-benzothiazole-6-carboxamide of formula (I) comprising: (a) oxidizing 2,5-dimethyl-benzenesulfonamide of formula (II) in the presence of an oxidizing agent to receive 2-sulfamoyl-terephthalic acid of formula (III); (b) converting the formed 2-sulfamoyl-terephthalic acid of formula (III) to receive 2-sulfonoyl-terephthalic acid derivative of formula (IV); and (c) reacting the resulting 2-sulfamoyl-terephthalic acid derivative of formula (IV) with ammonia (NH3) or ammonium containing-salt; wherein R represents OR′ or CI and R′ represents a branched or non-branched C1-C12 alkyl.