The most recent and detailed specifications can be found in IEC 61010-1 2010-06, Annex K. It's the first standard document that discusses horizontal and vertical insulation distances in PCB consistently.
As for minimal prepreg thickness, the document refers to standard test voltages, manufacturer voltage ratings or alternatively actual HiPot measurements and redundancy of insulation layers. E.g. have at least two prepreg layers, each rated for the applied voltage.