The UN food agency says it has had to start cutting rations to 4m people in Zimbabwe because of a lack of funds.
The World Food Programme said it had had no response to its October appeal for $140m (£91m) to feed Zimbabwe.
Food could run out entirely by January unless fresh support is provided, the WFP warned, as it takes six weeks for supplies to reach rural areas.
Up to half the 10m population may need food before the next harvest is expected in April, the agency said.
The WFP said it had cut each person's monthly cereal ration to 10kg (22lb) from 12kg, and the pulse ration to 1kg from 1.8kg, to make current stocks last longer.
WFP spokesman Richard Lee warned that the situation would become "critical" in many parts of the country in the coming months.
"We're already hearing stories about families living on at most one meal a day, children going into the bush to search for wild food and wild fruit," Mr Lee told the BBC's Focus on Africa programme.
"It's a very serious situation and we're still four-and-a-half months away from the next harvest."
The WFP fed 2m people in October, the first month of a large-scale aid project, but the figure is expected to rise to 5.1m by early 2009.
Once hailed as a model economy and a regional breadbasket, Zimbabwe's fortunes have nosedived since 2000 when President Robert Mugabe seized white-owned farms and handed them over to landless blacks with little experience in farming.
We are seeing large numbers of people across the country who simply cannot access enough food for themselves and their families
Mr Lee said drought and inadequate farming inputs were also to blame.
"Large numbers of people in rural areas harvested little, or nothing, this year because of a drought, because of a lack of seed, because of a lack of fertiliser and other inputs," he said.
Mr Lee said Zimbabweans in the towns as well as the countryside were experiencing problems.
"The economic collapse in Zimbabwe has affected people across the country, and also in urban areas, so we are seeing large numbers of people across the country who simply cannot access enough food for themselves and their families, and require international assistance to see them through to the next harvest in April 2009."
The country is also hit with inflation running at a staggering 231,000,000%, and a breakdown in basic services has led to deadly outbreaks of cholera in the capital Harare.
Western nations have said they are ready to release hundreds of millions of dollars in aid, but not while Mr Mugabe retains his grip on power.
Mr Mugabe and Zimbabwe's opposition leader, Morgan Tsvangirai, have agreed to form a unity government to revive the economy, but they cannot decide how to share out cabinet posts.
Last weekend, Mr Tsvangirai, leader of the Movement for Democratic Change (MDC), warned that at least one million people could starve to death in a year if the political deadlock continues.
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