France's new left-wing alliance robbed President Emmanuel Macron of his majority and is set to become the largest opposition force in parliament, but divisions over key issues may complicate keeping a united front.

Macron's Ensemble (Together) coalition emerged as the largest party in Sunday's National Assembly vote but fell short of a majority, a result portrayed as a huge win by a left that made major gains.

"The rout of the presidential party is complete," said former Marxist Jean-Luc Melenchon, leader of the NUPES coalition that along with its allies won 137 seats out of the 577 up for grabs, according to an AFP count based on interior ministry figures.

All members of the NUPES benefited from the alliance, with the hard-left France Unbowed (LFI), Greens and Communists increasing their share of seats, while the Socialists avoided a wipeout despite a very poor showing in the April presidential vote.

But amid a surge for Marine Le Pen's far right party National Rally (RN) the left-wing coalition also failed to come close to obtaining a majority of its own, which would have helped inter-party cohesion.

Macron's setback means the centrist president will be forced to look for allies to push through his reformist agenda, and may be looking to win over MPs from the moderate left.

"We will try and convince the moderates who are present in the parliament, even if there aren't many," government spokesperson Olivia Gregoire told France Inter radio Monday.

"The rout of the presidential party is complete," said former Marxist Jean-Luc Melenchon, leader of the NUPES coalition
"The rout of the presidential party is complete," said former Marxist Jean-Luc Melenchon, leader of the NUPES coalition AFP / BERTRAND GUAY

LFI MP Mathilde Panot on Monday flatly ruled out the left-wing alliance working with Macron. "We (have) extremely different world views," she told RTL radio.

While the left alliance became the main opposition force, the far-right National Rally became the largest single opposition faction in the National Assembly as a standalone party.

Sunday's results prompted Melenchon on Monday to propose that the four parties of the NUPES merge into one group at parliament.

"If there is one group, without any discussion the opposition would be called NUPES," Melenchon said.

But the Socialist leader at the National Assembly, Valerie Rabault, immediately slapped down the proposition.

Projected number of seats in the incoming French parliament by party or alliance, according to full results from the Interior Ministry, and breakdown of the outgoing parliament
Projected number of seats in the incoming French parliament by party or alliance, according to full results from the Interior Ministry, and breakdown of the outgoing parliament AFP / Valentin RAKOVSKY

"The left is plural, it is represented in its diversity at the National Assembly ... Wanting to get rid of this diversity is a mistake and I'm against it," Rabault said on Twitter.

The Greens and Communist Party also announced they would not support the move.

The NUPES -- or New Ecological and Social Popular Union -- was the first left-wing tie-up in 25 years and gave new hope to the left, which had been in the political wilderness since 2017.

After narrowly missing out on the second round of the April presidential vote, Melenchon reached out to other forces on the left in the hope of thwarting Macron's pro-business agenda.

The bloc campaigned on a platform to lower rather than raise the retirement age, to increase the minimum wage and levy taxes on the rich.

But differences over the European Union and nuclear energy that were swept under the carpet during the campaign may bubble to the surface if Macron is forced to piece together a majority to pass each bill.

"There are people among the Greens and the Socialist Party who are still interested in working with Macron if they can have some sort of leverage on policies," said Martin Quencez, a researcher at the German Marshall Fund.

Sciences Po Grenoble professor Simon Persico said the fragmentation of the parliament, and in particular the presence of 89 far-right National Rally MPs, will give an incentive to the left to stay united, even if they do not merge into one group.

It will be vital for the bloc to avoid "being in a logic of division and to continue sending the message that the presidential camp is isolated", Persico added.

For now, the left alliance has agreed on a coordination group that will lay the ground for common positions and help deal with tensions behind closed doors.

"That may be enough in the short run," said Quencez. "It's not like the government, which needs to show unity -- they can still disagree and most of the time vote against anything Macron proposes."

And in an indication of the determination to keep the alliance alive beyond the parliamentary polls, Greens secretary general Julien Bayou tweeted: "What we've built doesn't stop here.

"For ecology, for social justice and for democracy -- count on us," he added.