(Adds details and statement)
PARIS, April 1 (Reuters) - The French Greens party will not take part in a newly formed government under Socialist Prime Minister Manuel Valls, it said on Tuesday, breaking away from a coalition marked by tensions over energy policy.
The decision capped nearly two years of tense relations between coalition partners as the Greens sought to wind down use of nuclear energy and insisted on maintaining a blanket ban on shale gas exploration.
It followed a rout of the ruling Socialist Party in local elections on Sunday which prompted an unpopular President Francois Hollande to appoint his tough-talking former interior minister Valls as prime minister.
"The ecologists will back the government without fail when it acts for progress and ecology, but it will oppose it whenever ecology is not taken into account," the party said in a statement.
The party's executive committee voted not to join the government against the opinion of most of the party's 18 members of parliament, two parliamentary sources told Reuters.
The Greens' departure leaves President Francois Hollande's Socialist Party and allies still with a majority in parliament, suggesting the government should have enough support to pass a new budget later this year.
Two Greens party ministers, one of whom had clashed openly with Valls, had said they would not join his government despite his offer to create an expanded Ministry of Ecology.
(Reporting by Chine Labbe; writing by Nicholas Vinocur; editing by Astrid Wendlandt)