China should immediately end its one-child policy and instead adopt a two-child scheme, a foundation with close links to the highest levels of the Communist party has said.
The Chinese government currently limits most urban couples to one child, and allows two children for rural families if their firstborn is a girl.
The report from the China Development Research Foundation set a three-year deadline to phase in the reform.
"We have been discussing the one-child policy since 2000," said Li Jiamin, one of the co-authors of the study. "Making the jump to two children is only a matter of time now."
He added: "If China sticks to the one-child policy, we are looking at a situation as bad as the one in southern Europe. Old people will make up a third of the population by 2050."
Another co-author, Cui Fang, is an economist who heads the Population and Development department of the Chinese Academy of Social Sciences.
Prof Cui said he was unable to comment because the report had leaked out ahead of the 18th Communist Party Congress, the highly sensitive moment when China will unveil a new set of leaders for the first time in a decade.
However, in March, he argued that the government should keep the promise it made to the public three decades ago, when it introduced the one-child policy.
"They said if the circumstances changed, so would the policy. We should realise this promise," he said.
"In the past, family planning was important for our national development, but now the country has changed and the decision about how many children to have should be given back to families," he added.
The Chinese population has been reduced by between 100 million and 400 million, according to various estimates.
However, the new study argues that bringing the policy to an end will not unleash a huge population boom.
It pointed to four experimental areas in northern and central China which had their family planning controls lifted in 1985.
"All these areas had low population growth, and the birth rate has been shrinking since 2000. But the gender balance in these areas is better, and so is the age distribution."
The report admitted, however, that some other experimental areas had seen births spiral "out of control" and had family planning policies reapplied. (©Daily Telegraph, London)