French MPs have effectively voted to relax the Socialist-era 35-hour limit on the working week, allowing private firms to increase working hours.
Unions say the move effectively dismantles the 35-hour week
The National Assembly, dominated by the centre-right, voted by more than two to one to allow up to 13 hours' overtime.
Private sector workers will also be able to convert extra days off into wage rises or pension contributions.
Employers said the 35-hour week, introduced in 1998, had failed to create jobs and was uncompetitive.
The new law does not scrap the 35-hour week - it still applies in France's large public sector and it will remain the standard working week in the private sector.
But the changes will allow workers to work up to 48 hours a week - the maximum allowed by the European Union.
Labour Relations Minister Gerard Larcher hailed the reform as a "pragmatic and realistic" move, but Socialist deputies dismissed it as a backward step, the Associated Press news agency reports.
"This text in fact just shows the blind refusal of a ruling party that is heading for trouble and toughening its line on ideological principles that date from another century and penalise jobs and workers," Socialist deputy Alain Vidalies said.
Those in favour of changes, such as lorry drivers, were a minority
A poll earlier this year showed that the majority of French workers did not want to work longer hours, with only 18% saying they did.
Public sector trade unions mobilised against the reforms, bringing hundreds of thousands of protesters on to the streets in a series of protests across the country.
But many blue collar unions said members wanted more pay, not more time off.
The 35-hour week was introduced by a Socialist government, in the expectation that it would help reduce unemployment.
However, unemployment remains stubbornly high at 10%.
Prime Minister Jean-Pierre Raffarin said the changes were aimed at restoring the work ethic in France and improving its sluggish economic performance by encouraging people to earn more by working more.
He said the change was vital to keep the French economy competitive and to create more jobs.