1. A method for the marking of documents, said documents consisting in digitized images of black and white, grey and/or colour types, wherein a digitized mark image is intimately mixed together with the original digitized image by means of a digital operation producing a modified digital image which occupies the same portion of surface as the original image, each grey or colour value in the modified digital image being expressed as a function of the grey or colour values of the original image and of the mark image, in such a way that the mark is not noticeable in the modified image to the human eye but appears on the copy of this original document produced by means of a photocopier, the digitalization of the original document being effected with a sampling pitch at least twice finer than that of the photocopier.