The South American country, a key source of wheat for neighboring Brazil, controls exports of the grain through a quota system designed to guarantee affordable local food supplies and help tame high inflation.

This season, the government announced it would free up six million tonnes for shipment, but months of wet weather have flooded fields in key wheat-growing areas, and a fungus outbreak has worsened the production outlook.

Officials telephoned exporters last week to inform them of the decision to reduce this season's export quota to 4.5 million tonnes, an industry source told Reuters.

In a meeting late on Monday, government officials and wheat exporters agreed to limit shipments in January and February to a total of 2 million tonnes.

Findings from a new audit of stocks and production will determine whether more shipments will be allowed in March, the export industry source said, adding that previously granted export permits had not been officially withdrawn.

Daily newspaper La Nacion said some 5 million tonnes of export permits had already been approved, but only 4 million would now be allowed, meaning exporters would effectively have to reduce shipments by a further 20 percent.

So far, exporters have shipped about 500,000 tonnes of the 2012/13 wheat crop, which Buenos Aires Grains Exchange expects to come in at 9.8 million tonnes.