The cause of a power outage that delayed 12,000 passengers in Japan on Sunday has been found.

A singular slug has been blamed for the power failure on rail lines in south Kyushu on May 30.

The outage took place during peak travel times, at 9.40am, which forced rail company JR Kitakyushu to cancel a total of 26 trains, reports CNN.

During an inspection of the rail network, engineers found a dead slug measuring just under three centimetres long.

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The tiny slug had burned to death after touching an electrical cable leading to the mass power failure, according to a company spokesman. The slug had got in through a gap in the power box.

"We often have trouble with deer colliding with trains but not a problem with slugs," he added.

JR Kyushu states that it has checked similar devices in the area and confirms there are no other slug intrusions.