One of the two leaders of the French ecologists' parliamentary group has quit the Green Party (EELV), accusing his former comrades of ultra-leftism and defending the alliance with President François Hollande's Socialists, reports RFI.
The resignation is not the party's first and seems unlikely to be the last, according to media reports.
After being a member of EELV and its precursor Les Verts for 20 years, François de Rugy announced his resignation on Thursday and, in a longstanding French political tradition, published a book explaining the move on the same day.
Called Ecology or ultra-leftism, you have to choose, it slams the Greens' shift to the left since they left the Socialist-led government in 2014.
De Rugy is angry at a recent decision to ally with Jean-Luc Mélenchon's Left Front, which includes the Communist Party, in some parts of the country in December's regional elections.
He is a longstanding partisan of alliances with the Socialists, an unpopular stance with the rank and file since the party saw its vote evaporate while it had two ministers in Jean-Marc Ayrault's government.
De Rugy worked with Ayrault on the local council of Nantes, western France, for several years and was part of the Greens' negotiating team with his successor as prime minister, Manuel Valls.