Use of N (p toluenesulfonyl) N′ (3 p toluenesulfonyl oxy phenyl)urea with an X ray diffraction pattern with Bragg angles (2θ/CuKα) of 10.3, 11.0, 12.9, 13.2, 15.4, 17.1, 18.0, 18.2, 19.4, 20.0, 20.7, 21.2, 23.0, 24.9, 25.3, 26.5, 26.8, 27.5, 30.7, 32.7 as a colour developer in a heat sensitive recording material comprising a carrier substrate, a heat sensitive colour forming layer which is applied to one face of the carrier substrate and which contains at least one non phenolic colour developer and at least one colour former, and an adhesive layer and/or a coating in order to allow rear face printing using conventional printing methods on the carrier substrate face facing away from the heat sensitive colour forming layer, so as to limit the loss of image density and/or the relative print contrast and/or the reduction of the area based colour developer quantity, wherein, with a value of ≥1.20 optical density units for the heat sensitive recording material stored in accordance with the migration test defined in the description, the image density equals at least 35% of the value of the image density prior to storage and/or the relative print contrast of the heat sensitive recording material stored in accordance with the migration test equals at least 70% of the value of the relative print contrast prior to storage and/or the area based colour developer quantity of the heat sensitive recording material stored in accordance with the migration test equals at least 30% of the area based colour developer quantity prior to storage.