French unions will keep on fighting against President Emmanuel Macron's pension reform plan despite the upper house of parliament approving the text on Saturday, a leader of one of the main unions said on Sunday.

Laurent Berger, secretary general of France's largest union the CFDT, also warned Macron's government against forcing the parliament's hand by pushing the text through without a vote from MPs of the National Assembly, using a procedure known as 49:3 after the related article in the French constitution.

"Given the mobilisation of the population, the level of opposition to the plan (...) you cannot resort to a democratic flaw by using this 49:3 procedure," he said on BFM TV.

"In my opinion, (using 49:3) would be very dangerous as it risks creating a great degree of bitterness," Berger added.

Now that the Senate has passed the pension reform, whose key measure is raising the retirement age by two years to 64, it will be reviewed by a joint committee of lower and upper house lawmakers, probably on Wednesday.