"While everyone has the right to a private life, when one is a public figure and president, one must be careful to avoid being ridiculous.
"Well, that photo of Hollande coming out of his mistress' place with a motorbike helmet makes Hollande look totally ridiculous. He is the ridiculous President."
Sarkozy's outburst came amid reports that Gayet is pressing charges for "violation of privacy" against the magazine that published photos it said proves she is Hollande's mistress.
Hollande expressed his "total outrage" at the publication in Closer. He admitted he was enduring "painful" times with Valerie Trierweiler, his current partner and de facto first lady.
But the President, who has not denied the affair, said he would not be pressing charges for violation of privacy as it would be seen as "double standards"given that he enjoys presidential immunity.
Trierweiler is in hospital due to "emotional shock". According to Le Canard, she had a "nervous breakdown" when Hollande broke the news of his affair.