France s Prime Minister Elisabeth Borne. AP
Raising the pension age is one part of a broad bill that is the flagship measure of President Emmanuel Macron's second term.
The bill is meeting widespread popular resistance, more than 1 million people marched against it earlier this month, and misunderstanding about what it will mean for today's French workers.
In an interview with France-Info radio broadcast Sunday, Prime Minister Elisabeth Borne said the age "is no longer negotiable.''
Retirement at 64 and a lengthening of the number of years needed to earn a full pension "is the compromise that we proposed after having heard employers' organizations and unions,'' she said.
A union-led online petition against the retirement plan saw a surge in new signatures after Borne's comments. France's eight leading unions are in discussions Sunday about a joint response to her remarks, according to officials with the FO and CFDT unions.
Lawmaker Manuel Bompard, whose France Unbowed party is leading the parliamentary push against the reform, called for "the biggest possible'' turnout for upcoming strikes and protests.
"We have to be in the streets Tuesday,'' he said on BFM television Sunday.
The government says the reform is necessary to keep the pension system solvent as France's life expectancy has grown and birth rates have declined.
"Our aim is to ensure that in 2030 we have a system that's financially balanced,'' Borne said.
Unions and left-wing parties want big companies or wealthier households to pitch in more to balance the pension budget instead.
Borne suggested openness to adjustments on how the reform addresses time that people take out of their careers to bear children or pursue education. The plan's critics say women are unfairly targeted; Borne disagreed, but said, "We are in the process of analyzing the situation.''
The bill goes to a parliamentary commission on Monday, and to a full debate in the National Assembly on Feb. 6. Opponents have submitted 7,000 proposed amendments that will further complicate the debate.