Japan's population is expected to shrink to a third of its current size over the next century, with the average woman living to over 90 within 50 years, a government report said Monday.

The population is forecast to decline from the current 127.7 million to 86.7 million by 2060 and to tumble again to 42.9 million by 2110 "if conditions remain unchanged", the health and welfare ministry said in the report.

The projections by the ministry's National Institute of Population and Social Security Research forecast that Japanese women would on average have just 1.35 babies, well below the replacement rate, within 50 years.

The report said that last year's earthquake and tsunami in northeastern Japan, which left more than 19,000 people dead or missing, hit average life expectancy but that the figure was expected to continue its upward trend.

Japan's life expectancy -- one of the highest in the world -- is expected to rise from 86.39 years in 2010 to 90.93 years in 2060 for women and from 79.64 years to 84.19 years for men.

In September the government announced that the number of people aged 100 or older hit a record high for the 41st consecutive year.

The health ministry said 37 out of every 100,000 people in the country are now centenarians -- a total of more than 47,700, with 87 percent of them women. The figure was more than 3,300 higher than in 2010.

More than 20 percent of Japan's population are aged 65 or over, one of the highest proportions in the world.

Japan's population has been declining as many young people have put off starting families, seeing it as a burden on their lifestyles and careers. A slow economy has also discouraged young people from having babies.

Analysts say having a smaller population is not in itself a problem, as demonstrated by the economic and diplomatic successes of many European nations with far fewer people than Japan.

But an ageing population causes all manner of difficulties, most notably for Japan's government finances, already hard pressed by two decades of economic stagnation.

More retirees inevitably means more spending on social security when Japan's public debt, at twice GDP, is already one of the industrialised world's worst.