• The defecting MPs want to focus on green issues and social inequality.
• But their decision leaves Mr Macron's party with 288 seats, one short of a majority in the 577-seat lower house.
The party of French President Emmanuel Macron has lost its outright majority in parliament, after a group of MPs broke away to form a new party.
Ecology, Democracy, Solidarity will be largely formed of seven MPs from La République en Marche (On the Move) and other ex-supporters of the president.
The defecting MPs want to focus on green issues and social inequality.
But their decision leaves Mr Macron's party with 288 seats, one short of a majority in the 577-seat lower house.
French commentators said La République en Marche (LREM) still had the backing of two other political allies, the centrist MoDem as well as Agir from the centre-right, which together make up another 56 seats in the National Assembly.
There is even a chance that the party could regain its absolute majority if another defector who leaves the assembly is replaced by a pro-Macron MP.
The Macron camp has been plagued by a series of defections in recent months, and French media said the seven latest departing MPs had come under intense pressure to stay.
Two MPs who had originally planned to join EDS backed down at the last minute, Le Figaro reported.
The new party is not a major blow to Mr Macron electorally, and allies dismissed the moves as part of the "tribulations of parliamentary life".
But it is further evidence of dissatisfaction among the president's MPs, who were swept into the National Assembly in June 2017, weeks after his whirlwind presidential victory.