China should gradually increase the retirement age as part of a solution to fill a possible future pension fund gap, according to a green book on population and labor released on Wednesday.

The green book, released by the Chinese Academy of Social Sciences, proposes a timetable for increasing the retirement age.

It suggested that all female employees retire at the age of 55 starting from 2018, and the age will then be increased by one year every three years until 2045, said the green book.

The retirement age for male employees, who can now retire at 60, will be increased by one year in every six until 2045, when both women and men will retire at the age of 65, the book said.

"Increasing the retirement age can effectively slow down a long-term decline in the labor force, and is helpful in boosting social equality," said Du Yang, a professor at the Institute of Population and Labor Economics.

"It is reasonable to postpone the retirement age first by increasing that of women, as a woman in her 50s today is at the prime of her career and is capable of working, and women's life expectancy and their educational level has increased in recent years," said Su Hainan, deputy head of the China Association for Labor Studies.

In China, men can retire at 60, while women who work in factories can retire as early as 50. Female public-sector workers can retire at 55.

By the end of last year, there were around 210 million people in China aged 60 or above, accounting for 15.5 percent of the population. The proportion is expected to increase to 38.6 percent in 2050, according to Office of the China National Committee on Aging.

Even though there isn't a significant deficit in pension funds so far, some provincial governments may be unable to meet their obligations to retirees in the near future, according to Su.

While experts advocate delaying the retirement age to help the country cope with its increasing elderly population, some citizens prefer not to stay longer in the workforce.

"I would rather retire early and earn more money by operating a restaurant on my own," said Wang Xiaoyong, a cabdriver in Beijing. "Working another five years won't really make a difference to my income."

Su suggested that different approaches should be taken in different sectors.

"There will be more detailed plans for people to adapt to this multi-year process," Su added. "Public servants could take the lead, and then people in other sectors could follow."

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