With just four days before the first round of voting, former town planning minister Fadela Amara announced she would vote for front-runner Hollande.
Adding to Sarkozy's woes, the incumbent slipped five points behind his Socialist rival for the first round of voting, compounding the huge 16-point lead Hollande already has in the May 6 runoff.
Speaking on French television, Sarkozy dismissed the latest poll results.
There's not much point in commenting (on the polls) when they're good and then commenting on the others because they're bad, he said, according to Reuters.
Amara joins a growing list of defectors, including former environment minister Corinne Lepage, high commissioner on poverty Martin Hirsch, equal opportunities junior minister Azouz Begag and former culture minister Jean-Jacques Aillagon.
The news comes a day after it emerged former conservative French President Jacques Chirac said he would vote for Hollande in Sunday's presidential elections instead of his successor and one-time protégé Sarkozy.
Jean-Luc Barre, an author and friend who helped Chirac write his memoirs, told the newspaper Le Parisien on Monday that Chirac's June statement that he would rather vote for a socialist than Sarkozy was not a joke, despite the French media's coverage of it as such.
Jacques Chirac is true to himself when he says he will vote for Francois Hollande, Barre said, according to the Telegraph.