UPDATE 4-Macron, Abe to discuss Renault-Nissan at G20 as feud brews

* France's Macron requested meeting with Abe at G20 - Mainichi

* Japan minister sent France rare letter of protest - paper

* Authorities approve extension of Ghosn's detention - media (Adds Renault union comment)

TOKYO/PARIS, Nov 30 (Reuters) - French President Emmanuel Macron will discuss the Renault-Nissan alliance's future with Japan's prime minister at the G20 summit in Argentina on Friday, seeking to defuse a brewing diplomatic row over the balance of power inside the partnership.

The auto alliance is facing the biggest test of its 19-year existence after the ousting of Nissan Motor Co Chairman Carlos Ghosn, who was arrested last week for suspected financial misconduct.

Tokyo authorities on Friday approved the maximum 10-day extension of Ghosn's detention, Japanese media said. Prosecutors have to file charges against Ghosn, 64, by Dec. 10 or arrest him on suspicion of fresh crimes to keep him in custody.

Tokyo prosecutors declined to comment. Nissan did not immediately respond to a request for comment.

Ghosn's detention in Tokyo has left the Franco-Japanese auto alliance without its leader and main interlocutor with the French government, which owns 15 percent of Renault and wants to maintain the current alliance capital structure.

Renault controls Nissan through its 43.4 percent stake, while Nissan holds a reciprocal non-voting 15 percent stake in its French partner.

BUENOS AIRES MEETING

An official in the French presidency said Macron would discuss the alliance with Prime Minister Shinzo Abe at midday in Buenos Aires, but gave no further details.

Meeting in Paris last week, Japanese Industry Minister Hiroshige Seko and French Finance Minister Bruno Le Maire reaffirmed the two countries' support for the alliance.

A few days later, however, Le Maire said on French television that he and Seko had agreed that keeping the alliance's current capital structure was desirable - an agreement the Japanese minister denied making.

The Mainichi Shimbun daily reported on Friday that Seko had sent a rare letter of protest to Le Maire for the remarks.

Officials at Japan's Ministry of Economy, Trade and Industry said they could not immediately comment on the letter. A French finance ministry official said Le Maire's office had no comment.

Le Maire's apparent faux pas drew some criticism from Renault staff representatives concerned for the alliance.

"The government should know their place and stay there," said a union official at the French carmaker. "This kind of overreaching may be counter-productive."

Ghosn's arrest, including for allegedly under-reporting his income, has triggered new attempts by Nissan to shake off what it considers Renault's outsized control of it, adding to problems at Macron's Elysee.

As economy minister, Macron had masterminded the French government's surprise increase of its Renault stake in 2015, raising alarm bells inside Nissan that the Elysee was out to wield more influence over the Japanese company.

The auto alliance, which also includes Japan's Mitsubishi Motors Corp, for its part "emphatically reiterated" its commitment to the partnership on Thursday after executives met in Amsterdam for the first time since Ghosn's arrest.

A review of the capital structure was not discussed at the meeting, Mitsubishi Motors' CEO Osamu Masuko said. (Reporting by Kaori Kaneko, Chris Gallagher and Linda Sieg and Elaine Lies in TOKYO and Michel Rose and Laurence Frost in PARIS; Writing by Chang-Ran Kim; Editing by Muralikumar Anantharaman/Keith Weir/Jane Merriman)