Japanese authorities have hit former Nissan boss Carlos Ghosn with a fresh charge of aggravated breach of trust, the fourth formal indictment against the auto sector tycoon, who immediately filed for bail.

The Tokyo District Court confirmed that prosecutors had filed the new indictment and hours later said that Mr Ghosn's crack legal team had filed a request for the executive to be released on bail.

The 65-year-old strenuously denies all allegations against him and insists they have been cooked up in a "plot" by Nissan executives wary of his plans to bring the Japanese car giant closer to its French partner Renault.

According to the formal charge sheet, Mr Ghosn is accused of funnelling millions in Nissan funds to a dealership in the Middle East and syphoned off around five million dollars for his personal use.

The transfers were made "with the purpose of benefiting himself by receiving part of the money," alleged prosecutors.

Nissan itself said it had filed a criminal complaint against its former boss "after determining that payments made by Nissan to an overseas vehicle sales company via a subsidiary were in fact directed by Ghosn for his personal enrichment and were not necessary from a business standpoint."

Experts believe these are the most serious charges yet against Mr Ghosn since he was dramatically arrested on 19 November as he landed in his private jet at a Tokyo airport.

Mr Ghosn has already won bail once before - but under strict conditions such as agreeing not to leave the country and living under surveillance.

When he last won bail, Mr Ghosn walked out of the detention centre in front of the world's media dressed like a Japanese labourer with a cap and a face mask in an apparent attempt to give reporters the slip.

The rollercoaster case of the executive, once revered in Japan for saving Nissan from the brink of bankruptcy, has gripped the business world and shone a spotlight on the Japanese legal system that has come in for some criticism especially from abroad.

Japan's justice system allows authorities to keep suspects in custody for prolonged periods and trials almost always result in a conviction - sparking outrage from outside the country.

Today was the end of the maximum period authorities had to question Mr Ghosn, who is now technically in pre-trial detention.

The Ghosn case represents a precipitous downfall for an executive who has gone from a life of luxury and private jets as he ran three huge car companies to a small cell.

Nissan says an internal investigation uncovered "substantial evidence of blatantly unethical conduct" by the former chairman.

He has already been stripped of his position on the board at Nissan and resigned from the head of Renault and the three-way alliance the two companies share with Mitsubishi Motors.

His wife Carole has also been questioned by prosecutors in Tokyo.

She was allegedly president of a company registered in the British Virgin Islands which received some of the five million dollars moved from Nissan funds to go towards a luxury yacht.

Carole Ghosn has also denied any wrongdoing and been active in the media in recent days, writing an opinion piece in the Washington Post calling on US President Donald Trump to lean on Japanese counterpart Shinzo Abe to allow her husband to obtain bail.

Mr Ghosn's lawyers have expression reservations that he will receive a fair trial as 99 percent of all cases that come to trial end in a conviction.

Carole Ghosn has also said she is concerned about her husband's health in conditions in the Japanese jail that Mr Ghosn himself said he would not wish on his "worst enemy."

His lawyers in Paris have urged the French government to insist he be tried in France as a fair trial would "only be possible" there, according to them.