Australians have been told to pray for rain or face a ban on irrigation in the main food-growing region so that there is enough water to drink.

The Prime Minister’s warning yesterday heralded a dramatic increase in food prices and the prospect of tens of thousands of farmers having to watch their crops fail.

John Howard said that an expert panel had advised the Government that the worst drought in the nation’s history left it no choice but to turn off irrigation systems in the agricultural heartland of the Murray-Darling basin in the east.

Its 55,000 farmers supply virtually all of Australia’s vegetables, stone fruits, citrus fruits, cotton and rice. It is also home to many of its vineyards. Food prices are expected to rise immediately and there were predictions last night that scores of farmers would be forced to walk off their land. Winemakers said that the 2008 vintage would be crippled.

Mr Howard said that only the unlikely event of huge rains within the next six weeks would replenish the Murray-Darling river system, Australia’s largest inland water source.

Years of drought have devastated many small towns as farm incomes have shrunk, but Mr Howard said that the experts had made clear that the situation was now “unprecedentedly dangerous”. If water supplies were not shut off to farmers, it would be impossible to guarantee that people in inland towns and cities would have enough water to drink or wash.

Jolyon Burnett, head of the Irrigation Association of Australia, said: “If it continues like this we will see food becoming increasingly scarce and it will be reflected in the price of it. Annual crops simply won’t be planted.”

Ben Fargher, head of the National Farmers’ Federation, said supplies of stone fruits, grapes, avocados and almonds would be seriously affected for years. Once trees died, it would take four or five years for replanted trees to produce fruit. Winemakers predicted that the 2008 vintage would be even worse than this year’s, which has suffered a 40 per cent drop in the grape harvest.

Joy Sutton, who farms with her husband on the border of New South Wales and Victoria, said they would abandon their stone fruit trees and that many other farmers would give up.

The past 12 months have been the driest in 115 years of record-keeping for flows into the inland river system. Restrictions on the use of water have long been in place in Brisbane and Melbourne, including a ban on washing cars, extremely limited watering of gardens and an expectation that people will take four-minute showers.

Over the years of drought, old towns flooded decades ago to create dams have re-emerged and at least two old murder cases have been re-opened as dried-up lakes have revealed discarded weapons.

Mr Howard has long refused to link the Australian drought with global climate change and he did not change his position yesterday. He said the drought was “a national challenge to be dealt with at a national level” and he wants the states to agree a A$10 billion (about £5 billion) national water plan to make supplies more secure.

But Julia Gilliard, the deputy leader of the Australian Labor Party, said that unless Australia began to deal with its contribution to global climate change, future drought and water supply crisis would worsen. A Labor government would sign the Kyoto protocol on climate change. The Howard Government has refused.

Mr Howard’s plan includes upgrading pipelines, making farm irrigation systems more efficient and improving storage systems. But yesterday his appeal went higher: “We must all hope and pray there is rain,” he said.

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