A method of joining the edges of flexographic printing plates on printing cylinders, which includes the following steps:(i) fixing the printing plates to make-ready cylinders by means of a first double-sided self-adhesive film having the same area as the printing plate,(ii) cutting the printing plates to size so that they fit the printing cylinders,(iii) removing the cut printing plates from the make-ready cylinders and from the first self-adhesive films,(iv) fixing the cut printing plates by means of a second double-sided self-adhesive film, which has the same area as the printing plates and from 1.6 to 2.6 times the thickness of the first self-adhesive films, so that defined gaps result between the edges of the fixed printing plates,(v) filling the gaps between the edges to be joined, with a liquid photopolymerizable mixture of binders, photopolymerizable monomers and photopolymer, ization initiators and(vi) partially polymerizing the mixture under a transparent film cover by exposure to UV light until the elongation at break in the composite consisting of the solid gap closing compound and the printing plate material fulfils the conditionR.sub.S