Ernest-Antoine Seilliere, head of the MEDEF main business federation, said the conservative government must carry out its promises to ease the 35-hour rules swiftly to make French industry more competitive.
Business chiefs have welcomed a pledge by President Jacques Chirac to allow more flexibility over the working week but have criticised him for saying the law itself must stay. Unions have vowed to fight any changes to the law.
"Currently we don't work enough in France to be competitive. The whole world knows it," Seilliere told RTL radio. "We have an enormous budget deficit and quite weak growth - so come on! We must carry out the reforms we need quickly and well."
He said France ranked only 62nd in the world in terms of hours worked annually and that he was urging Prime Minister Jean-Pierre Raffarin to give employers more freedom within the law to negotiate with employees over how much they work.
"We have something very precise to say - make it possible for companies to hold collective or individual negotiations to decide the working time," he said. "This is fundamental for us".
His comments reflected frustration among French business leaders that the government has not yet done more to put its promises over the 35-hour working week into action.
Raffarin said last week the 35-hour week introduced by the Socialists before his government took power in mid-2002 had slowed economic growth and he pledged to make the fight to stop the flow of jobs abroad a top priority.
He said the French should be able to work longer hours if they want to and more flexible rules should be agreed at the latest by the end of 2005.
The shorter working week was supposed to increase employment but the jobless rate is running at almost 10 percent, compared with a European Union average of 9.0 percent.
Several government ministers said last week that growth could reach 2.5 percent this year but Seilliere was more cautious, saying the rate could be closer to the government's initial target of 1.7 percent.
Faced with high labour costs, some firms are shifting production away from France to lower-cost countries. Opponents of the 35-hour working week say rolling back the rules would help attract employers, create jobs and generate additional tax revenues to ease the strain on France's public finances.
France is set to bust the European Union's deficit limit for the third year running this year.
Some French workers have already begun to accept longer working hours as the price for holding on to their jobs.
Workers at a Robert Bosch car parts factory in France voted last month to put in more hours for the same pay to save jobs. They were the first employees to vote to scrap the 35-hour week.
Bosch said the plan would prevent production moving to the Czech Republic, where labour is cheaper.
Germany is also trying to stem a flow of jobs abroad. Workers at manufacturing and technology giant Siemens and at carmaker DaimlerChrysler have agreed to work longer hours in moves aimed at raising productivity.