A catalyst which, by means of a reduction reaction at ambient temperature, permits the elimination of nitrates and nitrites in waters. The catalyst comprises a combination of a noble metal and a non-noble metal supported on or incorporated into the structure of a support which, in elemental and anhydrous form, has the formula XYMgAl, in which X is at least one noble metal, Y is at least one non-noble metal, Mg is magnesium and Al is aluminium, the Mg and Al preferably forming the structure of a hydrotalcite or a mixed oxide deriving from a hydrotalcite.