Former French prime minister Mr Laurent Fabius has threatened to oppose the European constitution in a referendum next year unless President Jacques Chirac secured changes in European Union policy.
A "no" campaign led by Mr Fabius and his opposition Socialist Party would reduce the chances of France backing the constitution and could scupper the treaty altogether because it needs the approval of all 25 EU member states.
By opposing the constitution, Mr Fabius would go against the overwhelming majority of European Socialists and trade unions, and trigger a leadership battle in his own party.
He said he would approve the constitution only if Mr Chirac secured changes in EU policy that boost employment and prevent "delocalisations" - the migration of firms to other regions of Europe where taxes or costs are lower.
"I set one condition, and one only, for the president because it is he who is in charge of France's European policy. I demand a new policy be put in place to boost employment and fight against delocalisations," he told France 2 television in an interview.
"If the head of state follows these lines, which implies a big change of direction, and manages to convince our partners, it will be good for the workers and good for the [constitution] treaty. If he does not, the response will be 'no'."
Mr Fabius demanded changes to the EU Stability and Growth Pact - which underpins the euro - that would help boost employment. He also called for an increase in the EU budget to favour research and training that would boost jobs.
He demanded "fiscal harmonisation" - a levelling of taxes throughout the EU - to discourage firms from moving abroad, and called for stronger measures to guarantee public services.
These steps, he said, should be carried out before the referendum, expected in France in the second half of 2005.