Audio and visual out of sync when exporting

[Karl25008555aj0l]

When I export, my audio and visual are out of sync. The audio starts to speed up and go further out of sync. It does sync when I export as an mpg, but I have been asked to deliver as a H264. Any ideas please?

[Ann Bens]

If the footage is variable framerate convert to constant framerate with Handbrake or Shutter Encoder before bringing into Premiere.