The World Food Program says next week the daily ration will be reduced from 2,100 calories to 1,050 for 6.1 million people being supported by the agency throughout the country.
The normal ration is regarded as a minimum for healthy sustenance, especially for some three million people in Darfur who receive food from the WFP.
"It was not easy to take this decision, but we don't have any choice," WFP spokeswoman Christiane Berthiaume said.
The food agency had received only US$238 million (A$316 million) from western donor countries.
It had requested US$746 million (A$990 million) this year for Sudan.
Tough decision
WFP Executive Director James Morris said: "This is one of the hardest decisions I have ever made. Haven't the people of Darfur suffered enough? Aren't we adding insult to injury?"
In a statement, Mr Morris underlined that the shortfall in emergency relief aid for Sudan was "hard to understand" because countries had doubled their payments for longer term development assistance to 107 billion dollars in recent years.
"Donors are being incredibly generous -- but they are not putting victims of humanitarian crises like Darfur first on their list," he added.
The WFP is aiming to limit rations so that it can keep resources in reserve until September, to hold out during a critical bridging period ahead of the next harvest.
Violence continues
Civil war is raging in the western Darfur region, while in the south a difficult period of reconstruction is in progress after the end of two decades of conflict there.
Khartoum and rebels in Darfur are under intense international pressure to clinch a peace deal by Sunday.
Food aid will be essential to shore up any peace deal, Mr Morris insisted.
"Throughout this critical year for Sudan, when peace must be allowed to take hold, WFP urgently needs donors to come forward so that we can guarantee food aid to the millions of Sudanese who so desperately need our help," he said.
African Union mediators on Tuesday presented the warring parties in the devastated western region with a draft peace agreement and urged them to sign the deal by Sunday.
The conflict, which began in February 2003 when rebels rose up against the Khartoum government, has left some 300,000 people dead and forced at least 2.4 million to flee their homes.
Most of the surviving victims in Darfur rely on food aid, that was orginally shipped in amid the threat of famine. Last year aid helped to reduce the malnutrition rate, according to WFP.
However, the rate has increased again in recent months during the dry season, international aid agencies have warned.
"Food must come first -- we cannot put families who have lost their homes and loved ones to violence on a 1,000 calorie a day diet," Mr Morris said.
"This is a measure we should simply never have to take. Our donors were really supportive in 2005 -- they cannot be less so in 2006," he added.
Lack of security in the region is also preventing relief organisations from reaching about one third of those in need, the UN children's fund UNICEF said this week.
Nearly two thirds of the funding received so far for food deliveries comes from the United States, according to WFP data.
The next biggest country donor is Libya followed by Canada, Norway, Ireland, Italy, Switzerland and Belgium.
Bush pressures Sudan
US President George W. Bush pressed the government of Sudan to end the violence in the country's troubled Darfur region.
The call came ahead of nationwide protests to call attention to the continuing humanitarian crisis.
"The message to the Sudanese government is: We're very serious about getting this problem solved. We don't like it when we see women raped and brutalized," the president said at a White House press conference.
"We expect there to be a full effort by the government to protect human life and human conditions," the president said.
Mr Bush made his remarks ahead of a White House meeting with activists lobbying on behalf of victims of the violence in Darfur.