LONDON — More than 100 million people are being driven deeper into poverty by a “silent tsunami” of sharply rising food prices, which has sparked riots around the world and threaten U.N.-backed feeding programs for 20 million children, the top U.N. food official said Tuesday.

“This is the new face of hunger — the millions of people who were not in the urgent hunger category six months ago but now are,” the executive director of the United Nation’s World Food Program, Josette Sheeran, said at a London news conference. “The world’s misery index is rising.”

Prime Minister Brown, hosting Ms. Sheeran and other private and government experts at his 10 Downing Street offices, said the growing food crisis has pushed prices to their highest levels since 1945 and rivals current global financial turmoil as a threat to world stability.

“Hunger is a moral challenge to each one of us as global citizens, but it is also a threat to the political and economic stability of poor nations around the world,” Mr. Brown said, adding that 25,000 people a day are dying from hunger-related causes.

“With one child dying every five seconds from hunger-related causes, the time to act is now,” Mr. Brown said, pledging $60 million in emergency aid to help the WFP feed the poor in Africa and Asia, where in some nations the prices of many food staples have doubled in the past six months.

Mr. Brown said the “vast” food crisis was threatening to reverse years of progress to create stronger middle classes around the world and lift millions of people out of poverty.

Prices for basic food supplies such as rice, wheat and corn have skyrocketed in recent months, driven by a complex set of factors including sharply rising fuel prices, droughts in key food-producing countries, ballooning demand in emerging nations such as China and India, and the diversion of some crops to produce biofuels.

Ms. Sheeran noted that America, which she said provides half of the world’s food assistance, has pledged $200 million in emergency food aid and that Congress was considering an additional appropriation.

Holding up the kind of plastic cup that the WFP uses to feed millions of children, Ms. Sheeran told reporters that the price of a metric ton of rice in parts of Asia had risen from $460 to $1,000 in less than two months.

“People are simply being priced out of food markets,” she said.

The WFP has budgeted $2.9 billion this year — all from donor nations — to conduct its feeding programs around the world, including large efforts in Sudan, Somalia, Ethiopia, and other nations that could not otherwise feed themselves.

Ms. Sheeran said soaring prices mean that the WFP now needs another $755 million to meet its needs. That “food gap” jumped from $500 million just two months ago as prices keep rising, she said.

“We hope we have reached a plateau, but this is a rapidly evolving situation,” she said, adding that the WFP was urgently seeking contributions to make up the difference as the situation becomes more dire in poor countries such as Bangladesh and Afghanistan that are heavily dependent on imported food.

Ms. Sheeran said the WFP’s main focus was on the “ultra poor,” those who earn less than 50 cents a day. She said rising food prices meant millions of people earning less than $2 a day were giving up health care and education. Those living on less than $1 a day were giving up meat and vegetables, and those living on less than 50 cents were facing increasingly desperate hunger.

Hunger and anger have led to violence recently in Haiti, where food riots earlier this month resulted in several deaths, as well as Bangladesh, Burkina Faso, Ivory Coast, Cameroon, Egypt, Indonesia, and Senegal. Argentina’s attempt to control rising prices led to a strike by producers.

The WFP is already being forced to cut back on school-feeding programs that now serve 20 million children, Ms. Sheeran said. Without more emergency funding, she said, a feeding program in Cambodia would be eliminated and programs in such places as Kenya and Tajikistan would be cut in half.

“These are heartbreaking decisions to have to make,” Ms. Sheeran said.