France, the European Union's biggest grain producer, is headed for its smallest wheat harvest in three decades because of heavy rains and lack of sunlight, the agriculture ministry has said.

Wheat output is projected to fall by 30% from the 2015 harvest to 29.1m tonnes, France's weakest level since 1986, which was drought year, it said.

"The crops have suffered from too much water and too little light," the ministry's statistics bureau Agreste said.

"The flowering and the grain maturation happened in bad conditions," it said, adding that weather conditions had also helped diseases and pests to spread.

The farming region around Paris as well as eastern and northern France were most affected.

The drop in French production comes at a period of high output in other wheat-producing countries, which has led to a fall in world prices, adding to the woes of French producers.

"We are seeing unimaginably catastrophic situations" for farmers, the Orana farmers union warned last week.

Some farms were bracing for a drop of up to 75% in their usual turnover, it said.