Chirac has already let it be known that he would not attend the congress in Paris next Sunday at which the ruling Union for a Popular Movement (UMP) is due to triumphantly anoint Sarkozy as its candidate.
Villepin insisted he made the decision - seen as a direct snub to the interior minister - because it would have been inappropriate for him to vote as "the president of the republic has not announced his intentions".
"As head of government I am not in a position to participate in this vote," he said.
But he continued with an attack on Sarkozy's leadership of the party, suggesting he was unable to accept open debate. "You don't win by lining up a political family behind a candidate like green peas or sardines in a tin," he said.
Despite dismal popularity ratings, both Chirac and Villepin - who make no secret of their animosity towards Sarkozy - have refused to rule out running independently against him.
Jean-Louis Debre, another senior "Chiraquien", who is also the speaker of the National Assembly, said late on Friday that he would not back Sarkozy in the party primary.
"I want to know before making up my mind which candidates will be representing the right and centre," he said, suggesting the names of Villepin, Chirac or their ally Michele Alliot-Marie, the defence minister.
Although at 74 Chirac is thought highly unlikely to seek a third term, he sowed confusion last week with a series of combative speeches that commentators saw as an attempt to puncture Sarkozy's campaign, and perhaps lay the groundwork for a rival bid.
"Day after day, the Elysee is emptying the UMP chief's programme of its substance," commented an editorialist in Le Republicain Lorrain newspaper.
Outlining a bold five-year plan of tax cuts and social measures, Chirac repeatedly warned against Sarkozy's campaign call for a "clean break" with the past policies of the centre-right.
Both Sarkozy and Segolene Royal, 53, his Socialist rival, are promising to sweep aside many of the features left by Chirac's almost 12 years in power.
A close Sarkozy ally, Patrick Devedjian, said on Sunday he believed Chirac was still toying with the idea of standing, even though a new Ifop poll showed that four in five French voters think he should not run again.
But he voiced confidence that "once [Chirac] has understood that there is no hope for him, he will bring [Sarkozy] his support".
He said: "I think that reason will finally triumph."