The agricultural ministry announcement that the wheat harvest is only about a third of what is required, and that imports are held up by lack of hard currency, came as a deadline passed today for the last white farmers to leave their land or face prosecution for trespass.

The maize harvest is expected to be equally dire and price controls to contain hyperinflation have emptied the stores of most other foodstuffs. The World Food Programme says at least 3 million people - one in four of the population - will need food aid in the coming months. It describes hunger in some parts of the country, which used to be a food exporter, as "acutely serious".

Last week, the government said it plans to import 100,000 tonnes of wheat but acknowledged that a shipment of 35,000 tonnes was held up in Mozambique because of a shortage of hard currency to pay for it.

The agriculture minister, Rugare Gumbo, has blamed the food shortages on black farmers who have taken over formerly white-owned land.

"I am painfully aware of the widespread theft of stock, farm produce, irrigation equipment and the general vandalism of infrastructure by our new farmers," he said.

"I am disappointed that our new farmers have proved to be failures since the start of the land reform programme in 2000. In spite of all the support government has been pouring into the agricultural sector, productivity and under-utilisation of land remain issues of concern."

The ministry of agriculture has also blamed electricity shortages for the wheat shortfall, saying that power cuts have affected irrigation and halved crop yields per acre.

The power shortages are likely to continue. Mozambique has reduced electricity supplies to Zimbabwe because of a $35m (£17.1m) unpaid bill. Shortages of coal and spares for power stations and mining equipment have also hit electricity production and power cuts are now a regular feature of daily life.

Zimbabwe, once the world's second largest exporter of tobacco, has also seen production of its main cash crop nosedive, further undermining its ability to buy food from abroad. This year's crop is not likely to be much better than recent harvests, with many farmers saying that their seedlings have died for lack of irrigation.

Cigarettes are only available on the black market at many times the official price, and now cost more than marijuana - a cash crop that does not appear to have been severely affected by the crisis.

The government's admission that the land redistribution has failed to deliver the promised boost to food production coincides with a deadline for the last white farmers to vacate their land. The farms were nationalised last year and the handover to the state was set for today.

Any farmer remaining on their former land faces prosecution for trespassing on state property. About 50 farmers have already been summonsed by the courts.

White farmers say that senior ruling party, military and intelligence officials have been touring their former properties to lay a claim and that they have little confidence the land will be distributed among the poor as the government claims.

Zimbabwe's economic problems are likely to be compounded by a law passed last week that compels many publicly owned companies, including foreign firms, to sell a majority of their equity to black Zimbabweans.

Critics say the legislation amounts to expropriation because it effectively forces the companies to hand over half of their value by taxing them to raise the money to "buy" the 51% stake for black investors approved by the government.

The government has ignored the protests of some foreign investors, including South African banks and mining houses. With the collapse of tobacco production, mining is now the country's largest source of foreign currency.

Zimbabwe's minister of indigenisation, Paul Mangwana, said those companies that do not like it can "pack their bags and go".

"If they feel that we went into the bush [to fight against white rule] for them to enjoy our wealth then they can leave. We are talking about the total liberation of this country. I have no apologies for that," he said.

Last week, the International Monetary Fund said that it would not renew assistance to Zimbabwe until it adopts economic policies rooted in "reality". The IMF suspended dealings with Harare late last year.

President Robert Mugabe continues to blame his country's financial problems on what he calls British-led economic sanctions. The UK says that the sanctions, imposed by the EU and the US, target leading Zimbabwe officials and have no impact on the economy.