The decision infuriated farmers and France's far right and conservative parties but was welcome by environmentalists and left-wing parties.

The law would have permitted the return of acetamiprid -- a chemical known to be toxic to bees and other creatures but relied on by some European farmers.

A petition against the bill garnered more than 2.1 million signatures after lawmakers adopted it on July 8. Critics said it was rushed through parliament's lower chamber without a proper debate.

The Constitutional Council said in its ruling that a provision in the bill allowing exemptions to the ban of products containing neonicotinoid chemicals such as acetamiprid would violate France's environmental charter, which has constitutional status.

In 2020, the council had granted a temporary exemption to the ban, limited to the beet and nut sectors which suffer most from the insects that acetamiprid acts against.

But the ruling said the legislature had undermined "the right to live in a balanced and healthy environment" enshrined in the environmental charter.

- Judges' power -

"Neonicotinoids have an impact on biodiversity, in particular on pollinating insects and birds, as well as consequences for water and soil quality, and pose risks to human health," the council said.

The council did approve two other measures in the law: one allowing for the construction of water storage for agricultural purposes, and another raising thresholds at which pig and poultry farms require prior authorisation.

President Emmanuel Macron took "note" of the ruling and will enact the law in a form that takes the ruling into account, his office said.

But the main farmers' union, the FNSEA, slammed the decision as "unacceptable and incomprehensible".

Laurent Duplomb, the conservative deputy who introduced the law, said it would mean more imports of products containing acetamiprid and reduced French production.

Agriculture Minister Annie Genevard, who supported the reintroduction of acetamiprid, said on X that the government would not leave the sectors affected "without a solution".

Marine Le Pen, the leading figure in the far-right National Rally (RN), said the court's judges had "behaved like a legislature while they have no democratic legitimacy".

"The level of interference of constitutional judges is becoming a real problem for our democracy," said Laurent Wauquiez, parliamentary head of the conservative Republicans (LR) party.

Socialist Party leader Olivier Faure called the decision a "victory for ecology" however.

- Acetamiprid legal in EU -

Banned in France since 2018, acetamiprid remains legal in the European Union, and proponents said that some French farmers needed it to help them compete economically.

The insecticide is particularly sought after by beet and hazelnut growers, who say that they have no alternative against pests and face unfair competition.

A petition on France's lower-house National Assembly's website called the measure an "attack on public health".

And beekeepers have described the chemical as "a bee killer". Its potential effect on humans is also a source of concern, though the risks remains unclear in the absence of large-scale studies.

For some opponents, frustration stretched beyond environmental and health concerns to exasperation over the country's political deadlock.

One supporter of the petition against the law called it "democratic revenge" after Macron forced a controversial pension reform through parliament in 2023 and dissolved the national assembly last year, sparking political turmoil that resulted in a hung parliament.

In June, before the law's passage, several thousand demonstrators - including farmers, environmental organisations and scientists -- rallied across France calling for the bill to be withdrawn.

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