Beijing's relaxation of its hugely controversial one-child policy is an attention-grabbing first step, but it will have to usher in greater changes if China is to tackle its looming demographic timebomb, experts say.

According to a Communist Party announcement on Friday, couples in China will now be allowed to have two children if one of the parents is an only child.

Since the late 1970s, most couples in the world's most populous nation have been legally restricted to a single child, in an effort to control population growth.

"I would not be surprised if a year from now, we're going to see more and even a complete abandonment of the policy," said Wang Feng, a Chinese population expert and the director of the Brookings-Tsinghua Centre for Public Policy in Beijing.

"The government is testing the waters right now. They know that the policy will have to be gone. The policy serves nobody's interests," he said.

China's fertility rate -- the number of children a woman has in her lifetime -- currently stands at 1.5, academics estimate, far below the 2.1 needed to keep a population stable.

Wang, who has written several books on China's demographic changes, called the move the biggest change to the policy since it was first implemented and an important step toward its eventual dismantling.

But he estimated that, due to existing exemptions, it could affect only about 10 million more couples, a sliver of China's 1.35 billion population.

"It does not affect a lot of people," he said. "And it does not generate too many new births that would make a significant dent or boost in the Chinese demographic landscape."

Nonetheless it represented a start to "openly and decisively" phasing out the rule, he said.

Current exceptions to the policy include some rural families whose first child is a girl, ethnic minorities, and couples who are both only children.

The new change was only a matter of time, experts say, as the restrictions have contributed towards putting China on course to a demographic disaster.

Officials have long argued that the one-child policy -- which is estimated to have prevented 400 million births -- has been key to the country's economic growth and rising prosperity.

But China's working-age population declined last year for the first time since 1963, according to the National Bureau of Statistics, and the ranks of its elderly are swelling, with 30 percent of people expected to be over 60 in 2050, the United Nations says.

The proportion was only 10 percent in 2000.

"There's an economic reason (behind the move), because China now starts to worry that in 20 years or even less, there will be a labour shortage," said Cheng Li, director of research and senior fellow at the Brookings Institution's John L. Thornton China Center in Washington, D.C.

Experts point out that the fertility rate fall is not solely due to the one-child policy, as lower infant mortality levels have led to a global drop in fertility rates.

China's infertility rates have also quadrupled over the past 20 years, and the rising costs of rent, medical care and education mean that even couples who qualify to have a second child often decide against having one.

"I think it depends on the person," said Zhang Yan, 29, who was shopping in Beijing Saturday afternoon with her husband and two-year old son. "Some people might want to be able to have two kids, but these days many young people in China actually don't want a lot of children."

The couple are both only children so are already allowed a second child, she said, but have not yet decided whether to go ahead.

Given the exemptions to the rule -- the one-child limit already does not apply to around 37 percent of Chinese couples, according to a 2007 study -- as well as wealthier couples' ability to have a second child if they pay a fine, the impact will be limited, experts say.

"Over the past five or 10 years, the one-child policy has become increasingly irrelevant," said Li. "The middle class doesn't care about the policy because they can afford the penalty."

Even so, the childbearing restrictions have drawn condemnation abroad for their often brutal enforcement through forced sterilisation and late-term abortions.

Rights groups have long campaigned against the law and welcomed the move, but said it did not abolish the existence of the policy itself.

The use of coercion is the "basis of the system, and the reform that is being announced does nothing to address this," said Nicholas Bequelin, senior researcher at Human Rights Watch in Hong Kong.

"It is a step forward to the extent that some people are getting the ability to have a second child," he added. "But the policy still wrongly limits reproductive rights and is also still marked by many abuses."