BRUSSELS (Reuters) - Several influential EU states have dug in their heels on whether their farmers may grow one of Europe's oldest genetically modified (GMO) crops, raising the stakes in the EU's long-running stalemate over biotech policy.

The crop is a modified maize variety known as MON 810, marketed by leading U.S. biotech seeds company Monsanto.

Also known by its commercial name YieldGard, the maize type is designed to resist the European corn borer, a pest that attacks maize stalks and thrives in warmer climates in southern EU countries such as Italy and Spain.

While Monsanto says the protein contained in its maize has selective toxicity but is harmless to humans, fish and wildlife, an increasing number of the EU's 27 countries are unconvinced.

National GMO bans are the only part of Europe's biotech debate where EU countries can agree, since they see attempts by Brussels to order a government to lift its ban as an attack on national sovereignty. So, unusually, they tend to band together.

The European Commission has tried this on three occasions in the past two years and got a stinging rebuff on each occasion.

In the past few weeks, two EU agricultural powerhouse countries -- France and Germany -- entered the fray. Not only do they wield huge clout under the bloc's weighted voting system for decision-making, they also grow vast amounts of cereals.

First, Germany's government said maize produced from MON 810 seeds could only be sold if there was an accompanying monitoring plan to research its effects on the environment: a restriction that farmers say is tantamount to a growing ban.

The proposed restriction, to apply from 2008, has already been notified to Commission authorities in Brussels.

Soon afterwards, French government number two Alain Juppe, in charge of his country's environment, transport and energy policy, said in a newspaper interview that he would not exclude being "inspired" by Germany's proposed GMO ban.

Diplomats said it was too early to know if the French and German stances would affect voting for new GMO approvals -- EU countries have clashed over this for years -- but warned that it might alter the balance between 'pro' and 'anti' GMO countries.

"Even a national ban would get them into hot water with the Commission, but if it's a blanket change in position (on biotech policy) then it raises the stakes," one said.

NATIONAL BANS

Austria banned MON 810 maize in June 1999, around 14 months after the EU issued its original authorization. That national ban was cited, along with several others, by Argentina, Canada and the United States in an international challenge against the European Union at the World Trade Organization a few years ago.

Hungary, one of the EU-27's biggest grain producers, became the first eastern European country to ban GMO crops or foods when it outlawed the planting of MON 810 seeds in January 2005.

The same year, Greece and Poland used a provision in EU law that allows countries to decide whether to allow GMO seeds on national territory -- although a ban must be approved by EU member states to be legal. Both countries have restrictions in place against MON 810 maize.

Bulgaria's parliament has also indicated support for national restrictions for growing MON 810 maize.

EU environment ministers have slapped down several draft orders authored by the European Commission for countries to rescind their national GMO bans. This happened last February in the case of Hungary and also in December 2006, for Austria.

"We have had two councils (EU ministerial meetings) that have rejected Commission proposals (to lift GMO bans) with a large majority, and now there is this additional case in Germany," a Commission official told Reuters. "We have to look at the whole (GMO) authorization policy at some point."