More than a million people in Darfur will go without food rations by May unless new aid agencies are deployed, a joint Sudanese-United Nations assessment says.
It also says there could be major water shortages within two weeks.
The warning follows Sudan's expulsion of 13 large foreign aid agencies, mostly from Darfur.
Mr Bashir accuses them of spying for the International Criminal Court, which has issued an arrest warrant against him for alleged war crimes in Darfur.
Four of the expelled non-governmental organisations served some 1.1 million people, the report released on Tuesday said.
The assessment team toured Darfur from 11-19 March, and the report was co-signed by UN and Sudanese officials.
UN humanitarian affairs coordinator Ameerah Haq said that "the most critical needs are being filled for now".
"However, by the beginning of May, as the hunger gap approaches, and unless the World Food Programme has found partners able to take on the mammoth distribution task, these people will not receive their rations," she said.
The assessment also warned that "major water shortages could develop within two to four weeks, as from March 18, if fuel, incentives and spare parts are not continuously provided."
Since the expulsion of aid agencies, Sudan has said Sudanese groups have been filling the gaps, denying that there is any problem with the distribution of aid.
But UN humanitarian head John Holmes said the Sudanese government had not done enough, and that it had agreed in the report that gaps existed.