Allies of French President Emmanuel Macron signalled on Monday that he will push ahead with raising the retirement age despite a possible wave of strikes.
It came as unions warned Mr Macron against unleashing a “social conflict” by lifting the pension age from 62 to 65.
Several of France’s biggest unions were expected to meet on Monday to discuss the looming dispute.
But supporters of Mr Macron said his flagship reform was necessary to safeguard the welfare state.
“We cannot maintain a contributory pension system when we take on 100 billion euros of debt in the next few years,” said Finance Minister Bruno Le Maire.
Prime Minister Elisabeth Borne is expected to present a draft text to MPs on December 15.
Aurore Berge, the leader of Mr Macron’s party in parliament, rejected calls to water down the reform.
“We have to hold the line at 65,” she told France 3 television.
“We need a reform that is understandable, simple and fair.”
Mr Macron promised to raise the pension age during April’s presidential election campaign.
But his party lost its majority at parliamentary elections in June, meaning it will have to find support beyond his own ranks.
It will face robust opposition from left-wing MPs and unions who say it will add to cost of living problems.
Laurent Berger, the secretary-general of the French Democratic Confederation of Labour union, said workers would mobilise if the pension age is raised.
“I say this to the President and Prime Minister: listen to the world of workers,” Mr Berger said.
“Be careful not to create a great social conflict in January because you didn’t listen to the unions.”
The far-right National Rally, which has 89 seats, also opposes the pension reform.
Its former leader Marine Le Pen made it a key policy of her campaign against Mr Macron in April.
Philippe Ballard, a National Rally MP, said the funding gap for pensions was nothing compared to the debt incurred under Mr Macron’s presidency.
“Stop giving us lessons,” he said.