"I am deeply sorry to report that the situation remains intolerable and unacceptable," said the head of the Food and Agriculture Organisation Jacques Diouf as he presented its annual report.
When the leaders of 185 countries met at the World Food Summit in 1996, they pledged to halve the number of people suffering from hunger by 2015, to a maximum of 412 million.
"There are more hungry people in the developing countries today – 820 million -- than there were in 1996," Mr Diouf told a news conference.
"Far from decreasing, the number of hungry people in the world is currently increasing, at the rate of five million a year."
Unless the trend is reversed through "urgent and decisive action," the World Food Summit goal cannot be reached, Mr Diouf said.
He also warned that at the present rate the goal would not be reached until 2150.
"Fortunately, there is still time," Mr Diouf said, adding that to reach the goal the number of undernourished people must be reduced by 31 million per year until 2015. "It can, it must, be done."
Mr Diouf called on donors to honour commitments to boost overseas development aid to 0.7 percent of gross national product (GDP).
For their part, developing countries should step up investments in agriculture and carry out reforms "to create the right environment for foreign investment," he added.
The latest estimates by the FAO, show that 854 million people remain undernourished, which is defined as consuming fewer than 1,900 calories per day, in the period 2001-03.
Most, some 820 million, were in developing countries.