Experts fear military action would hit power supplies and transport routes, disrupting the flow of vital food supplies.
Care International, Christian Aid and Save the Children said a war without UN backing would make delivering aid even harder.
Raja Jarrah from Care International said: "Up to 10 million people will need feeding if the food ration system is disrupted.
"Upwards of 60 per cent of people face hunger, if not starvation.
"About 90 per cent of Iraq's sewage treatment stations are vulnerable if the electricity goes down, leading to polluted drinking water and dire public health consequences."
And he warned: "If a war does not have a United Nations mandate then it's going to be even harder for the United Nations humanitarian agencies to have access to the resources they need for the scale of the response required."
Mr Jarrah said there was "no evidence" that the international community is "prepared for that scale of disaster".