A process for the preparation of a compound of formula (I) wherein R1 is hydrogen, fluoro or chloro; which process comprises: a) reacting a compound of formula (II) wherein R1 is hydrogen, fluoro or chloro; with a nitration agent to the compound of formula (III) wherein R1 is hydrogen, fluoro or chloro; b) reacting the compound of formula (III) with trichloroisocyanuric acid in the presence of sulfuric acid or fuming sulfuric acid to the compound of formula (IV) wherein R1 is hydrogen, fluoro or chloro; and c) reacting the compound of formula (III) with chlorine gas at a temperature from 180° C. to 250° C. to the compound of formula (I).