By Maria Tsvetkova

MOSCOW (Reuters) - One of Vladimir Putin's main opponents may have broken a taboo by publishing what he says is the pseudonym used by one the Russian president's daughters to stay out of the spotlight.

Putin has made his and his family's private life little less than a state secret, keeping his rarely-photographed daughters Yekaterina, 28, and Maria, 29, out of sight and managing his divorce with the minimum fuss.

But opposition blogger Alexei Navalny on Thursday published on his Facebook page an online report which identified a certain Katerina Vladimirovna Tikhonova as the head of an organization working with Moscow State University.

A separate report on Wednesday by RBC, an independent multi-media holding, stated Tikhonova was among those heading a $1.7 billion project to build new University facilities but it did not make any connection between her and Putin.

Navalny wrote on his Facebook page: "RBC (they are cool!) yesterday found Putin's daughter in the Scientific Council of Moscow State University."

A source close to Moscow State University confirmed Tikhonova was Putin's daughter, telling Reuters on condition of anonymity: "Yes, it is her."

Tikhonova could not be reached for comment.

The Kremlin often brushes aside questions about Putin's private life, defending his right it privacy. Asked about the woman's identity, Kremlin spokesman Dmitry Peskov said: "I don't know who she is."

Asked whether Putin's daughter works for Moscow State University, he answered: "I don't know. It's not my job. I deal with the president, not with his children."

$1.7 BILLION PROJECT

RBC, which has a television channel, a newspaper and Internet portal, wrote on Wednesday about a development project under which Moscow State University, one of the country's most prestigious, would be expanded.

Under the headline "Who stands behind the MSU expansion", it wrote that Tikhonova was in charge of Innopraktika, an organization that was helping prepare the $1.7 billion project.

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RBC did not make any direct link to Putin, although it said several influential businessmen who are his allies were involved in the project, and said Tikhonova had declined an interview request. Innopraktika declined to comment to Reuters.

Sergey Aleksashenko, a former official at the central bank who is now an opposition figure, questioned in a blog whether it was correct for Tikhonova to hold such a role in a business project if she is indeed Putin's daughter.

"If this is true, it means that Vladimir Putin has crossed one of those 'red lines' he once drew for himself," he wrote.

Putin is so secretive about his family that most Russians have no idea what his daughters look like. It is also not public knowledge where his ex-wife Lyudmila is living since their divorce was finalised last year.

Putin did, however, say in November that his daughters both live in Russia and he sees them once or twice a month.

Media reports last year said Maria had lived in the Netherlands until Malaysia Airlines flight MH17 was shot down over eastern Ukraine, killing 298 people, many of them Dutch nationals. She fled after the local mayor called for her deportation, though he later retracted his comments.

Putin's spokesman denied rumors five years ago that Yekaterina planned to marry the son of a South Korean general.

Putin and Lyudmila announced their breakup to a television interviewer in June 2013 and they formally divorced in April last year after more than 30 years of marriage.

Putin said in 2008 there was no truth to a newspaper report that he was preparing to marry Olympic rhythmic gymnast Alina Kabayeva, who was born in 1983.

He told journalists at the time to keep their "snotty noses" out of his private affairs and the newspaper, Moskovsky Korrespondent, folded shortly afterwards.

(Reporting by Maria Tsvetkova, Writing by Timothy Heritage; editing by Janet McBride)