Abstract: The present invention relates to a solid composition comprising a water soluble chlorite salt, an oxidizing chlorine releasing agent, selected among sodium- or potassium-dichloro-s-triazinetrione or trichloro-s-triazinetrione, and a proton donor serving as a water soluble agent capable of lowering the pH of the aqueous solution to less than 3. The chlorite salt is advantageously selected among the sodium and potassium salts. The proton donor is advantageously selected among sodium- or potassium-hydrogen sulphates and pyrosulphates or among citric acid and malic acid. In said composition, the stochiomolecular ratio of the various components is suitably chlorite salt: oxidizing chlorine releasing agent: proton donor 4:1:3. The invention relates also to an aqueous solution in which the above composition is dissolved.