The latest calculations from the UN labour agency show that 250 million five-to-14-year-olds are employed, half of them full time. That's up sharply from earlier estimates of 73 million full-time child workers.
The new figures come after in-depth surveys and interviews in numerous countries. Previous estimates were based almost solely on official statistics.
The ILO report found nearly 153 million children are working in Asia, 80 million in Africa and 17.5 million in Latin America. It called for a new international accord banning the harshest forms of child labour: slavery, prostitution and work in hazardous industries.
Only 49 UN members ratified a 1976 child labour convention. Some nations said its limits on paid work were too broad.
Slavery or child bondage still is practised in South Asia, Southeast Asia and West Africa, the report said. Children are either sold outright or rural families are paid in advance by "contractors" who take children away to work in carpet weaving, glass manufacturing or prostitution.
Child trafficking for the sex industry is increasing despite better international awareness of the problem, the ILO said.
In Asia, child prostitutes number about one million and rising, the report said. Numbers also are increasing in Burkina Faso, the Ivory Coast, Ghana, Kenya, Zambia and Zimbabwe.
It identified international sex networks that take Latin American children to Europe and the Middle East, and Southern Asian children to northern Europe and the Middle East. Child sex markets also were well established in West Africa, Europe and the Arab world, it said.
Among other things, the ILO found that: certain industries are exposing their child workers to pesticide poisoning, lung diseases or even crippling their growing bodies by forcing them to carry heavy weights.
In Sri Lanka, more children die from pesticide poisoning than from a combination of other childhood diseases such as malaria, tetanus and whooping cough.
Children are exposed to dust and fumes in repair shops, woodwork factories and construction sites in Egypt, the Philippines and Turkey.
In glass factories, children are often forced to drag loads of molten glass from glowing furnaces amid noise levels that could cause deafness. Children as young as three were working in match factories where they were exposed to dust, asbestos and other hazardous fumes.
Up to five million child domestic servants work in Indonesia, including 400,000 in the capital, Jakarta.