PARIS, Dec 4 (Reuters) - France's largest farm union, FNSEA, urged farmers on Tuesday to refrain from buying genetically modified (GMO) seeds based on U.S. giant Monsanto technology ahead of a biotech law expected in February.

After an environmental policy conference in October, French President Nicolas Sarkozy said he would draft a new law on GMOs, notably on how farmers may grow the only GMO crop allowed in France, an insect-resistant maize developed by Monsanto.

The new law, due to launch a 'high authority' on GMOs, requiring farmers to declare their GMO plantings and making them financially liable for any contamination, was sent last week to the Council of State and is due to be adopted by Feb. 9. The Council of State vets French laws to ensure they conform with the constitution.

'In response to (the government's) willingness to do the right thing well and clarify matters, the FNSEA is asking farmers not to buy MON 810 GMO seeds before Feb. 9, 2008, when the parliamentary session is due to end and the vote on the bill,' the union said in a statement.

Sarkozy suspended the planting of GMO pest-resistant crops late October until the results of a review by the new authority on GMOs. The ban did not affect maize production in France as sowings do not take place until spring.

Other maize seed makers use Monsanto's technology, which is designed to resist the European corn borer, a pest that attacks maize stalks and thrives in warmer climates in southern EU countries.

Monsanto says the protein contained in its maize has selective toxicity but is harmless to humans, fish and wildlife.

Just 22,000 hectares -- 1.5 percent of France's cultivated maize land -- have been sown with GMO maize this year but some farmers have urged greater use of GMO crops to boost yields.