French unions have lost many of their traditional allies in parliament after Sunday's parliamentary election result, but President Emmanuel Macron cannot afford to ride roughshod over them if he is to successfully push through his reform agenda.

The size of his majority — already assured according to official figures and set to be over 350 seats in the 577 seat lower house according to projections — means Macron and his year-old centrist party can now count on lawmakers to give his government powers to wave the reform through without lengthy negotiations in parliament.

He has routed the left, yet he needs to handle France's muscular unions with care.

At stake is not only the injection of flexibility into the labor market he has promised. The way he handles his labor reforms will also set the tone for more reforms in the coming years, including a revamp of jobless benefits and retirement systems.

Though uneasy about Macron's program and fearful that it is being rushed, most unions are relieved his government has so far been careful to consult closely with them, and that it has promised to come up with detailed proposals in the coming weeks.

The question now is whether he will keep taking their input on board, said Jean-Claude Mailly, head of France's third biggest union, Force Ouvriere (FO).

"Is he going to say, 'you don't agree, well, I'm going to push (the reform) through by force'?," Mailly asked in an interview with Reuters.

"If so, one thing's for sure, his presidency is going to get off to a bad start on labour relations and there will be tensions," he added.

Even France's most reform-friendly and biggest private-sector union, the CFDT, refuses to rule out protests if its counter-proposals are ignored.

"We reserve the possibility like everyone to give our point of view democratically. If we have to hold protests, we will. If we have to mobilize in companies, we'll do it," CFDT head Laurent Berger told Reuters in an interview.