PARIS The head of France's Green party on Saturday threatened to withdraw support for the Socialist government unless President Francois Hollande made a clearer commitment to protect the environment.
The Greens have two ministers in the cabinet, at the housing and development ministries.
While the government does not rely on Green support to pass legislation, its loss would leave little room for dissent among the Socialists and their left-wing allies to get laws through.
Out of 577 seats in the lower house of parliament, the Socialists and their allies hold 292, while the Greens have 18.
The Green party has accused the government of dragging its feet on environmental policy after Environment Minister Philippe Martin put off plans for France's transition towards less carbon-intensive energy sources until next year.
It has also said it may not back the 2014 budget in parliament later this year after the government sent mixed signals about a tax break for diesel fuel.
Green party head Pascal Durand has given Hollande until an environmental conference on Friday to make a concrete commitment to France's energy transition.
"Does this government want, yes or no, the Greens by its side?" he said at a party meeting. "The answer is in its hands."
Former environment minister, Socialist Delphine Batho, was fired in July after she criticized the government's plans to cut her ministry's budget by 7 percent next year.
The Greens agreed at the time to stay in the cabinet, but said they would scrutinize the government's environment policies included in the 2014 budget, to be presented on September 25.
Party members were angered on Wednesday when Martin indicated that a tax break for diesel fuel would not be dropped, only to say later that he had been misunderstood. The government has since said no decision has yet been taken.
Diesel is widely used in French passenger vehicles, but the Greens say pollution from the fuel is responsible for thousands of deaths.
(Reporting by Leigh Thomas and Marine Pennetier; editing by Mike Collett-White)