A lawyer for Nicolas Sarkozy has denounced a decision to investigate the former French president's campaign funding, and says it will be appealed.
An investigative judge in city of Bordeaux summoned Sarkozy on Thursday and placed him under formal investigation for allegedly taking advantage of elderly L'Oreal cosmetic heiress Liliane Bettencourt in order to fund his 2007 presidential campaign.
Under French law, a formal investigation is the last step before a suspect is accused of a crime. If accused and found guilty, Sarkozy would face a maximum penalty of three years in prison and and a $484,000 fine.
On Friday, Sarkozy's lawyer, Thierry Herzog, called the judge's decision "incoherent" and "unjust."
The 90-year-old Bettencourt is the world's richest woman, according to Forbes magazine, which this year rated her and her family ninth on the list of the world's top 20 billionaires, with a net worth of $30 billion.
In 2011, a judge ruled Bettencourt, who suffers from dementia, be placed under the guardianship of her family.
Last year, investigators began looking into whether Sarkozy's campaign received hundreds of thousands of dollars worth of illegal contributions from Bettencourt, and police raided his home and offices in July. Sarkozy has denied the allegations.