France's upper house of parliament has passed a bill laying down conditions for the growth of crops using genetically modified organisms (GMO) after changing a key amendment aimed at limiting their cultivation.

The measure, passed by the upper house, or Senate, late on Wednesday, is a response to European Union demands that member states formulate laws on GMO use.

The bill has the backing of the ruling centre-right government and the main farmers' union, but has been fiercely criticised by campaigners opposed to the use of the technology.

It will return to the lower house of parliament, or National Assembly, in the second half of May before becoming law.

Under an amendment proposed by Communist deputy Andre Chassaigne to guard against contamination by GMO crops, the law makes it compulsory for farmers to 'respect agricultural structures, local ecosystems and non-GMO commercial and production industries.'

But a modification introduced in the Senate would leave it to a government-appointed High Council on Biotechnology to fix limits on what would constitute 'non-GMO' production for crop varieties, pending a ruling on the issue by the European Union.

Critics said the change would weaken the amendment but Greenpeace campaigner Arnaud Apoteker, an opponent of the bill, said the fact it had not been scrapped entirely was positive.

'We may have avoided the worst because ... the amendment was in danger and that was what we feared,' he said.

He said there was concern it could be further watered down when it returned to the National Assembly for a second reading.

As well as attracting condemnation from the left wing opposition, the GMO bill has caused deep divisions within the ranks of President Nicolas Sarkozy's centre-right government.

Junior Environment Minister Nathalie Kosciusko-Morizet, who favoured tighter restrictions, accused members of her own party and her own senior minister Jean-Louis Borloo of 'cowardice' over the issue.

(Reporting by James Mackenzie; editing by Chris Johnson)