Tens of millions of the world’s poor will have their food rations cut or cancelled in the next few weeks because rich countries have slashed aid funding.

The result, says Josette Sheeran, head of the UN’s World Food Programme (WFP), could be the “loss of a generation” of children to malnutrition, food riots and political destabilisation. “We are facing a silent tsunami,” said Sheeran in an exclusive interview with the Observer. “A humanitarian disaster is unrolling.” The WFP feeds nearly 100 million people a year.

Food riots in more than 20 countries last year persuaded rich countries to give a record $5bn to the WFP to help avert a global food crisis brought on by record oil prices and the growth of biofuel crops. But new data seen by the Observer show that food aid is now at its lowest in 20 years. Countries have offered only $2.7bn in the first 10 months of 2009.

The US, by far the world’s biggest contributor to food aid, has so far pledged $800m less than in 2008; Saudi Arabia has paid only $10m in 2009 compared with $500m in 2008; and the EU has given $130m less. Britain’s promise of $69m (£43.5m) this year is nearly $100m (£63m) less than 2008, and, if nothing more is given, will be its lowest contribution since 2001.

“Even under our best scenarios, we will end the year $2bn short,” said Sheeran. “Many of our funders do not feel that they need to give on the level of last year. They think the world food crisis is over, but in 80% of countries food prices are actually higher than one year ago.”

World food supplies are under increased strain this year following a succession of droughts, typhoons, floods and earthquakes that have destroyed crops in Africa and south-east Asia. But human needs are also greater because the financial crisis has led to widespread unemployment. In addition, the remittances from foreign nationals living in rich countries to their families at home are 20% lower than last year.