GENEVA (AP) _ France dealt a potentially devastating blow to crisis-ridden world trade talks Wednesday by announcing it would block European Community offers to cut import tariffs on farm products.

French Foreign Minister Roland Dumas said Paris regarded the proposals submitted by the EC's executive commission as ''null and void, and without (legal) commitment and value.''

After meeting Arthur Dunkel, director-general of the General Agreement on Tariffs and Trade, Dumas told reporters that the two European commissioners who submitted the proposals had violated their negotiating mandate.

The EC proposal was presented Wednesday at the 108-nation GATT talks. It gave the first detailed figures on cuts in EC import tariffs for individual farm products as part of international efforts to lower barriers to trade.

It is not strictly connected to last month's bilateral deal between the United States and the EC to cut farm subsidies. That deal averted the U.S. threat to impose 200-percent tariffs on European wine, and the prospect of an intercontinental trade war if the EC had retaliated.

But there is a linkage because France has so far blocked EC approval of the Washington accord and maintains that EC should not resume the international negotiations. Discussions among EC farm ministers on the issue ended inconclusively Tuesday.

France maintains the U.S.-EC subsidies deal exceeds support reductions agreed under the EC's internal farm reform program and that the EC executive commission had no right to negotiate it. Many French farmers fear they will be forced out of business if agricultural supports are cut. French President Francois Mitterand needs their support in next year's elections.

Dumas said he was surprised to hear that, despite French objections, the two EC commissioners had submitted the EC's detailed offers of tariff cuts for farm products at GATT headquarters.

Efforts to cut farm subsidies are part of the so-called Uruguay Round of trade talks, named after the country where they began. This is also meant to lead to a new agreement to liberalize trade in services like banking and insurance and to draw up new rules protecting copyright and patents from abuse.

The talks, launched in 1986, should have ended two years ago but were blocked by the U.S.-EC farm dispute. The Nov. 20 deal between the two trade giants boosted hopes that an overall accord was finally in sight.

GATT chief Dunkel set the end of the year as a target for overall political agreement, leaving technical details to be sorted out by the end of February. This is when the U.S. administration's special negotiating authority runs out.