Rising food prices could spark worldwide unrest and threaten political stability, the UN's top humanitarian official warned yesterday after two days of rioting in Egypt over the doubling of prices of basic foods in a year and protests in other parts of the world.

Sir John Holmes, undersecretary general for humanitarian affairs and the UN's emergency relief coordinator, told a conference in Dubai that escalating prices would trigger protests and riots in vulnerable nations. He said food scarcity and soaring fuel prices would compound the damaging effects of global warming. Prices have risen 40% on average globally since last summer.

"The security implications [of the food crisis] should also not be underestimated as food riots are already being reported across the globe," Holmes said. "Current food price trends are likely to increase sharply both the incidence and depth of food insecurity."

He added that the biggest challenge to humanitarian work is climate change, which has doubled the number of disasters from an average of 200 a year to 400 a year in the past two decades.

As well as this week's violence in Egypt, the rising cost and scarcity of food has been blamed for:

· Riots in Haiti last week that killed four people

· Violent protests in Ivory Coast

· Price riots in Cameroon in February that left 40 people dead

· Heated demonstrations in Mauritania, Mozambique and Senegal

· Protests in Uzbekistan, Yemen, Bolivia and Indonesia

UN staff in Jordan also went on strike for a day this week to demand a pay rise in the face of a 50% hike in prices, while Asian countries such as Cambodia, China, Vietnam, India and Pakistan have curbed rice exports to ensure supplies for their own residents.

Officials in the Philippines have warned that people hoarding rice could face economic sabotage charges. A moratorium is being considered on converting agricultural land for housing or golf courses, while fast-food outlets are being pressed to offer half-portions of rice.

The UN Food and Agriculture Organisation says rice production should rise by 12m tonnes, or 1.8%, this year, which would help ease the pressure. It expects "sizable" increases in all the major Asian rice producing countries, especially Bangladesh, China, India, Indonesia, Burma, the Philippines and Thailand.

Holmes is the latest senior figure to warn the world is facing a worsening food crisis. Josette Sheeran, director of the UN World Food Programme, said last month: "We are seeing a new face of hunger. We are seeing more urban hunger than ever before. We are seeing food on the shelves but people being unable to afford it."

The programme has launched an appeal to boost its aid budget from $2.9bn to $3.4bn (£1.5bn to £1.7bn) to meet higher prices, which officials say are jeopardising the programme's ability to continue feeding 73 million people worldwide.

Robert Zoellick, president of the World Bank, said "many more people will suffer and starve" unless the US, Europe, Japan and other rich countries provide funds. He said prices of all staple food had risen 80% in three years, and that 33 countries faced unrest because of the price rises.

In the UK, Professor John Beddington, the new chief scientific adviser to the government, used his first speech last month to warn the effects of the food crisis would bite more quickly than climate change. He said the agriculture industry needed to double its food production, using less water than today.

He said the prospect of food shortages over the next 20 years was so acute it had to be tackled immediately: "Climate change is a real issue and is rightly being dealt with by major global investment. However, I am concerned there

is another major issue along a similar time-scale - that of food and energy security."