France's top administrative court on Thursday gave the government a nine-month deadline to take "all the necessary steps" to reach its targets on climate change or face possible sanctions, reports Radio France Internationale.
The Council of State said France was currently set to miss its goal of reducing emissions by 40 percent by 2030 compared with 1990 levels.
Ruling on a case brought by the low-lying northern coastal town of Grande-Synthe, it ordered Prime Minister Jean Castex to take "all the necessary steps to curb greenhouse gas emissions" by March 31, 2022.
The deadline falls in the final weeks of campaigning in France's presidential election, meaning that Emmanuel Macron, who is expected to seek a second term, will be under intense scrutiny on an issue shaping up as a key theme in the vote.
If at the end of the nine-month period the council considers the government to still be falling short, it could impose hefty fines.
Former environment minister Corinne Lepage, who represented Grande-Synthe in the case, hailed the ruling as "historic."
"The noose is tightening on the government," the Affaire du Siecle (Case of the Century) campaign group, which includes Oxfam France and Greenpeace France, tweeted.
Despite his 2017 promise to "make our planet great again" -- a swipe at former US President Donald Trump, a global-warming denier -- Macron has been criticised for failing to meet France's targets under the landmark 2015 Paris Agreement on climate change.
In 2018, the popular climate campaigner Nicolas Hulot quit as Macron's environment minister over what he saw as the president's failure to take sufficient action on climate change.
The case lodged by Grande-Synthe, a town of 23,000 people built on land reclaimed from the sea that risks being flooded by rising ocean levels, is part of a mounting drive by activists worldwide to use courts to pressure governments into action.
In a ruling February that was also hailed as historic by campaigners, a Paris court found the state liable for its failure to take sufficient measures to meet its climate targets and ordered it to pay a symbolic one euro in damages.
In 2019, the Dutch Supreme Court ordered the Netherlands to slash greenhouse gas emissions by at least 25 percent from 1990 levels by the end of 2020, after a case brought by an NGO.