Mr. Sarkozy spent the day being questioned by investigative judges and was notified of the preliminary charges of “illegal financing of an election campaign” because he had, as a candidate, “exceeded the legal limit for electoral spending,” the Paris prosecutor’s office said in a statement.
Mr. Sarkozy leads the conservative center-right Republican Party, which was previously named the Union for a Popular Movement, known as the U.M.P. for the initials of its name in French.
The U.M.P. is suspected of covering spending by Mr. Sarkozy’s 2012 campaign that illegally exceeded the strict limit set by French law for presidential campaigns. In 2012, that limit was 22.5 million euros, or about $25 million in today’s dollars.
Bygmalion, a public relations company, is suspected of issuing false invoices to the U.M.P. for about €18 million, to bill for events that were actually organized for Mr. Sarkozy’s campaign.
Mr. Sarkozy has repeatedly denied that he knew about the dual accounting and the overspending, for which he has already paid a fine. Thirteen people, including executives at Bygmalion and aides close to Mr. Sarkozy, have already been placed under formal investigation in the case.
The opening of a formal investigation does not mean that Mr. Sarkozy will stand trial, and the charges can still be dropped, as charges were three years ago in another case when he was accused of manipulating a rich heiress into financing his first presidential campaign, in 2007.
But the new case is an obstacle facing Mr. Sarkozy’s bid to return to power. He is expected to run in the 2017 presidential election, and he is currently lagging behind his main opponent, former Prime Minister Alain Juppé, in polls for coming primaries on the right.
Mr. Sarkozy’s name has surfaced in several other cases, but he has been placed under formal investigation in only one of them, on charges of corruption and influence peddling. He has not been found guilty of wrongdoing or convicted in any of the cases so far.
On Tuesday, Mr. Sarkozy was also assigned the status of “assisted witness” on separate charges of fraud, embezzlement and falsification of records in what has become known as the “Bygmalion affair.”
The “assisted witness” status suggested that judges had not found any proof that Mr. Sarkozy was directly involved in wrongdoing on those charges, but judges can later decide to place him under formal investigation if new evidence comes to light in the case.