Jean-Pierre de Margerie, WFP country director based in Pyongyang, said the nation "may suffer deeper and more widespread hunger this year".
"Families and especially vulnerable persons will suffer from lack of access to food, eat fewer meals and have a poorer diet, increasing their vulnerability to diseases and illness," de Margerie said.
Banbury called on international donors to increase aid and urged North Korea to allow aid agencies to check that food is reaching the needy.
South Korean officials said in February that some of Seoul's rice aid intended for hungry civilians had been diverted to frontline army units.
The communist state suffered a period of famine in the late 1990s, which killed hundreds of thousands of people. It has relied on international aid since, but in 2006 ordered the WFP to drastically cut back its operations.
The UN agency said severe floods last summer had brought increased urgency to the longstanding problem of chronic food shortages.
It said prices of staples in the capital Pyongyang had doubled over the past year, with one kilogram of rice now costing a third of an average worker's monthly salary.
The UN's Food and Agricultural Organisation forecasts a 2008 food shortfall of 1.66 million tonnes, a near doubling of the 2007 deficit and the highest since 2001.
Paul Risley, WFP Asia spokesman, said North Korea had told the WFP in 2006 to cut back its programme following two good harvests in succession.
North Korea's government "knows full well the dangers of food insecurity",e said.
South Korea is the largest donor to the current WFP programme in North Korea, supplying $20 million. In recent years it has also provided around 400,000 tonnes of rice annually in bilateral aid.