Malawian President Bingu wa Mutharika has declared a national disaster over the food shortages which are threatening almost half the population.
Donors are being urged to prevent a repeat of the Niger crisis
In a radio and TV broadcast, the president said the crisis had worsened and the country needed more help.
UN estimates suggest about five million people will need aid after Malawi's worst harvest for more than a decade.
Mr Mutharika had been criticised for denying reports of deaths from hunger-related illnesses in Malawi.
Mr Mutharika said all 28 districts of Malawi were affected by shortages.
Malawi needed an extra 144,000 metric tons of food on top of 300,000 tons expected to be imported from South Africa, he said.
"The food crisis has escalated and we need more assistance," the Malawian leader said.
"We are facing a national disaster affecting the lives of our people."
The shortages are blamed on poor rains, a lack of seed and fertiliser during the planting season, and the effects of HIV and Aids.
Mr Mutharika's announcement follows increasing criticism from the opposition and church leaders, who urged him to do more to highlight the problem.
Correspondents say it comes as a dramatic turnaround for a president who three weeks ago said his government was in control.
Donors needed now
The president's declaration was welcomed by the UN World Food Programme.
A spokesman for the agency said it reflected the growing seriousness of the problem.
The hope now is that donors will be more generous in their response to a WFP appeal for $88m for Malawi, says the BBC's African affairs editor Grant Ferrett.
The deputy head of the agency, Sheila Sisulu, complained during a recent visit to the country that donations tended to be made only when images of large numbers of emaciated children began appearing on television screens around the world.
She said Malawi need not suffer the sort of severe hunger experienced earlier this year in the west African state of Niger, but warned that donors should come forward now.
It is not just Malawi which is threatened - across southern Africa, the UN estimates that 12 million people will need help in the coming year.