Mr Chevenement, 59, the founder-president of the populist Mouvement des Citoyens, was said to be in a deep coma after a severe reaction to an anaesthetic during vascular surgery on Wednesday.
As one of the principal Euro-sceptics of the left and leader of a populist faction closely allied with the Socialist Party, he occupies a pivotal position in the pink-red-green coalition that has been governing France since June last year.
He angered many people on what he dismissively called the "moral left" by pushing successfully for restrictive policies in immigration and law and order. In one of the key internal compromises in Lionel Jospin's government, he was given a free hand to take a tough line at his ministry in return for stifling his own anti-European Union views and supporters.
His loss would force Mr Jospin to make an awkward reshuffle and could destabilise what has been an unexpectedly successful administration.
The Mouvement des Citoyens, launched in 1992, is rightward leaning on social issues and leftish on economic issues. The group regards itself as much closer to French working people than the Socialists proper. Mr Chevenement played an important role in the campaign against the ratification of the Maastricht Treaty in 1992.
Yesterday Mr Jospin said the Minister for Overseas Territories and Departments, Jean-Jack Queyranne, would take over as interior minister for the time being.Reuse content