TOKYO, May 4 (Reuters) - As Japan prepares to mark its Children's Day national holiday on Saturday, an official survey showed the proportion of the population aged below 15 had fallen to a record low.

A rapidly ageing population has raised concerns that Japan will face a pensions crisis or labour crunch in coming years.

There were 17.38 million children in Japan as of April 1, accounting for an all-time low 13.6 percent of the entire population and marking the 26th straight year of declines, according to a government estimate released on Friday.

Kyodo news agency said Japan's proportion of children in the population was the lowest in the world, ranking behind 14.1 percent of Italy and Germany. Ethiopia had one of the highest percentages of children at 43.2 percent, Kyodo said.

The number of children in Japan has been lower than the number of people aged 65 or above since 1997, and a government think-tank has estimated their share in the population will fall to 12.3 percent, roughly half of that of the elderly, by 2013.

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