BEIJING (AFP) - China's top legislative committee on Saturday formally approved a loosening of its one-child policy, state media reported.

The decision was taken by the standing committee of the National People's Congress, China's rubber-stamp parliament, at the conclusion of a six-day meeting, according to Xinhua news agency.

The decision came just days after the standing committee had expressed support for the move following promises by the ruling Communist Party at its Third Plenum meeting last month.

Legislative approval was formally required to put it into effect.

The move seeks to widen existing exceptions to the one-child policy to allow couples where either parent has no sibling to have two children.

The family planning policy was imposed more than three decades ago to prevent overpopulation in the world's most populous nation.

China argues the limit kept population growth in check and supported the country's rapid development that has seen it soar from mass poverty to become the world's second-largest economy.

But enforcement of the policy has at times been excessive.

The public was outraged last year when photos circulated online of a woman forced to abort her baby seven months into her pregnancy.

Now China faces looming demographic challenges, including a rapidly increasing elderly population, a shrinking labour force and male-female imbalances.

China's sex ratio has risen to 115 boys for every 100 girls, while the working population began to drop last year, Xinhua said earlier.

The birth rate has fallen to about 1.5 since the 1990s, well below the replacement rate, it added.

But while the loosening - estimated to apply to around 10 million couples - has been welcomed critics say that the state has retained the principle of deciding itself how many children people should have.

The widely expected move came after the standing committee on Tuesday had expressed support for the changes.

The Third Plenum meeting has historically been an occasion for the ruling party to expand reforms, and was the first such gathering since Xi Jinping took over as head of the party in November last year as part of a once-a-decade change in power.

The party also pledged at the meeting to reduce the scope of the death penalty "step by step" - China is the world's biggest judicial executioner - accelerate reforms to the household registration system and loosen controls on the economy by giving markets a "decisive" role in the allocation of resources.