A United Nations summit on resolving the world's food crisis opened this morning with a call from Ban Ki-moon, the UN Secretary General, for world farm production to rise by 50 per cent by 2030 to meet growing demand.

He called on leaders to lower export restrictions and import tariffs on food with immediate effect to avoid further hunger and malnutrition, which have caused riots in several Third World countries.

The three day summit, organised by the Rome-based UN Food and Agriculture Organization, follows massive food price rises over the past three years. Analysts blame the diversion of crops to make biofuel, drought and natural disasters, fuel costs, and speculation.

Ed Schafer, the US Agriculture Secretary, claimed that biofuels are responsible for only 2-3 percent of the predicted 43 per cent rise in food prices this year. America has invested heavily in biofuel technology. Other summit participants however said that biofuel accounted for 15-30 per cent of food price increases.

In a message to the summit this morning, Pope Benedict XVI said that hunger and malnutrition are "unacceptable" in a world that has enough resources and know-how to end hunger.

The summit has been overshadowed by controversy over the presence of Robert Mugabe of Zimbabwe and Mahmoud Admadinejad of Iran, both of whom are due to address the gathering today. Neither has been invited to a banquet this evening hosted by Silvio Berlusconi, the Italian Prime Minister, at the Renaissaince era Villa Madama, which is used for Italian government receptions.

Under EU sanctions imposed on him because of his human rights record Mr Mugabe is not allowed to set foot in the EU, but the restrictions do not apply to UN meetings in Europe, Italian officials said. They said he had been granted entry "for the purposes of the summit only", as he had been for the FAO world food summit in 2002 and the organisation's sixtieth anniversary celebrations in 2005.

Douglas Alexander, Britain's International Development Minister, who is attending the Rome summit, said that Mr Mugabe's presence was "obscene", and pledged that he would not meet him or shake his hand. "This is a man who has impoverished his own nation, a country that was previously regarded as the bread basket of Africa and now has four million of his own population reliant on food aid......He neither has the credibility nor the authority to speak on issues of food production and prices."

Jewish leaders, the Italian left and Iranian exiles have denounced Mr Ahmadinejad's attendance at the meeting after the Iranian President repeated his call for the destruction of Israel before leaving Tehran for Rome.

Mr Alexander said Britain supported the World Food Programme's efforts to provide immediate humanitarian assistance as a short term solution.

In the medium term it wanted "a more effective functioning agricultural market" and in the long term a significant rise in agricultural growth and productivity.

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