Jet ski riders are being warned to avoid allowing the powerful streams of water propelling their craft along to enter into their anus after a woman died horribly after falling off the back of one.

Japanese accident investigators have reported two incidences of serious injuries incurred from personal water craft use.

In one 2011 summer accident, a Hyogo prefecture woman was riding on the back of the craft and fell off into the water due to the speeds attained.

The jet of water propelling the craft along streamed into her anus, causing fatal internal injuries.

In another similar incident in the same month in nearby Osaka, a woman suffered severe injuries in the same manner, but survived.

In both cases the craft’s operating instructions recommended the use of a wetsuit, but the women were only wearing swimwear.

The investigators warn that serious injuries could also be sustained should the jet enter other vulnerable orifices such as the mouth or ears, and advise caution in the event of a fall from such a craft.