France's ratification of the proposed European constitution will look more challenging than ever on Thursday night if Laurent Fabius, a former prime minister and a leading figure in the opposition Socialist party, uses a television interview to call on his supporters to vote “no” in next year's referendum.

The poll promises to be the main event in the 2005 political calendar in France and pollsters say Jacques Chirac, the president, has little chance of winning it without socialist support. Mr Fabius is seen as holding the balance of power in determining the party's position, and is expected to tilt against the constitution.