Some of the pesticides involved were originally put in makeshift stores because they were banned for use.
In a new report the UN Food and Agriculture Organisation (FAO) says almost 500,000 tonnes of old and unused toxic pesticides have been abandoned on sites.
The Rome-based organisation said the build-up of toxic pesticides that have been banned or expired is dramatically higher than previous estimates of around 100,000 tonnes.
The report is to be discussed at a meeting of international donors in Rome on Thursday and Friday.
According to the FAO, the quantities of obsolete pesticides in Africa and the Middle East are estimated at over 100,000 tonnes, in Asia at over 200,000 tonnes and in Eastern Europe and the former Soviet Union at more than 200,000 tonnes.
The FAO is still preparing inventories for Latin America.
Much of it is said to be accumulating in fields, farms and villages, and is poisoning the surrounding areas.
Child risk
"The lethal legacy of obsolete pesticides is alarming and urgent action is needed to clean up waste dumps," FAO expert Alemayehu Wodageneh said.
"These 'forgotten' stocks are not only a hazard to people's health, but they also contaminate natural resources like water and soil.
"Leaking pesticides can poison a very large area, making it unfit for crop production."
The problem is particularly acute in developing countries, where, the organisation says, people often prepare food and children even play amongst the rusting containers.
Some of the pesticides involved were originally put in makeshift stores because they were banned for use.
But the report says they are now leaking into the water supply and soil, contaminating large areas.
So far there has been no systematic study of the effect this is having on local populations.
But the UN says many of the substances are known to be intensely toxic and people living close to the stores routinely complain of nausea and headaches.
It says an urgent clean-up is needed - and is urging pesticide producers to help because, the UN believes, they have the knowledge, the money, and the responsibility to organise safe disposal.