[morzh]

As an EE, who did design a few commercial PCBs, including an off-line power supply for India Telecom, to another EE, this is what you do: it is called "creepage and clearance". You want clearance. This is simply achieved by cutting slots in your PCB, which replaces the surface creepage by the air gap, which immediately increases the breakdown voltage.
If you build your PCBs online, they are fine with slits; you could define them as a board outline or as a long oval unplated pad.