"I would not hide. This situation looks like a collective suicide," Juppe said, as quoted by Le Parisien newspaper, adding that he would "stay loyal" but was "ready" to serve as a replacement candidate.
On Wednesday, in the run-up to Fillon's press conference, the French media speculated on whether he might announce Juppe as his substitute. Instead, Fillon reaffirmed his determination to run despite the scandal around his wife and stressed that the investigation into his family's finances was "a political assassination."
The center-right candidate said that he would attend the interview with the examining magistrate he had been summoned for and would "tell the truth." Fillon also reiterated that he was not guilty of misappropriating public funds.
The scandal broke out in January around Fillon's wife allegedly having been employed as his parliamentary assistant and paid without actually working.
Juppe lost to Fillon in the second round of the center-right primaries in November 2016, with 33.51 percent to Fillon's 66.49 percent.
Juppe served as the prime minister under Jacques Chirac in 1995-1997. In 2002-2004, he was the president of Union for a Popular Movement (UMP), later rebranded as The Republicans. Juppe held a number of positions in Sarkozy's government and is now the mayor of the city of Bordeaux.
