The split was a history-setting precedent that struck a deep personal blow to Sarkozy's young presidency.
In a terse, 15-word statement, Sarkozy's office said Thursday the two were separating by mutual consent and would not comment further on the decision. Sarkozy's spokesman said separation meant divorce. The Sarkozys are the first French presidential couple to separate while in power.
The split comes as Sarkozy faced his first major political challenge: strikes Thursday that hobbled transport nationwide.
Sarkozy has remained unfazed by the strikes. He has not given any hint that his marital troubles will dent his determination to push ahead with his ambitious program of economic, political and social reforms for France.
In the past week, as speculation about his marriage reached fever-pitch, he continued to present an image of business as normal. He was to be in Portugal on Thursday for an EU summit.
Nicolas and Cecilia Sarkozy split for a few months in 2005, and she had seemed ill at ease as first lady since her husband's election in May. She did not cast a ballot in the runoff, and has rarely appeared with her husband in public in recent months.
Her one political venture came back to sting her: She raised her profile dramatically during a July mission to seek the release of five Bulgarian medics and a Palestinian doctor jailed in Libya. The stunned French media questioned her diplomatic credentials, and parliament is investigating arms deals signed soon after the release.
"She was shaken, murdered, wounded by the controversy," Isabelle Balkany, a friend of the couple, said on France Inter radio Thursday.
"Cecilia is a woman of conviction who needs to do things, feel useful. She knew that she would have trouble tolerating the conventional side" of being a president's wife, she said.
Balkany predicted the split would have no effect on Nicolas Sarkozy's job.
Even if he is "affected to his depths" by the "painful" decision, she said, "I sincerely think that it will have absolutely no impact on his mission as chief of state."
Cecilia Sarkozy accompanied Sarkozy through the recent years of his political career, acting as an aide, confidante and an ever-present figure at political events.
Dynamic and ambitious, the two set out to buck conventions in French politics, her in designer denim and him jogging and speaking in straight, inelegant sound bites.
Their separation, too, sets a precedent, and sets Nicolas Sarkozy apart from France's past leaders.
Their idyll was shattered in 2005, when photos of Cecilia hand-in-hand with another man on a Manhattan sidewalk were splashed across a magazine cover.
Sarkozy talked about it on national television, saying: "Like millions of families, mine has experienced some difficulties."
Observers wondered then whether Sarkozy could become president without her support and presence beside him. The question turned out to be moot, as she came back in time for Sarkozy's presidential push.
News reports Wednesday said the couple had seen a judge to say they are seeking a formal separation, prompting debate among experts over whether the constitution allows them to divorce because of the strong legal protections designed to keep sitting presidents out of court.
Both Sarkozys have been previously married. They have two children each from their previous marriages, as well as their own son, Louis.
Until the Sarkozys, French presidents' private lives remained largely private affairs. But Sarkozy courted the spotlight for years in his long run-up to the presidency - and that has meant his marital troubles were front-page news.
"The couple was quite extraordinary, really fused together," said Christine Clerc, political journalist and author of "Tigers and Tigresses," a book about presidential couples in modern France.
"They became an ordinary couple, a couple like many others who don't get on well anymore after 14 years, but who have a child in common, who had many projects in common, who have a deep bond, and who have lived many things together," she said.
The daily Liberation, for example, devoted five pages and its front page to Sarkozy's marriage on Thursday, relegating the strikes to inside pages - even though the newspaper traditionally leans left and might have been expected to devote more attention to the labor unrest had the presidential couple not overshadowed it.
Protesters at a union-led march in Paris on Thursday had little sympathy for Sarkozy's personal woes.
"There are problems more serious than that," said Yvelle Franck, a 63-year-old marching to protect retirement benefits.