Carlos Ghosn will remain in detention for another 10 days after a Japanese court approved a prosecutors’ request to continue questioning the former Nissan chairman over allegations of financial misconduct and breach of trust.

Ghosn, who has been held at a Tokyo detention centre for more than six weeks, is accused of underreporting his salary by tens of millions of dollars and of transferring personal investment losses to the automaker he rescued from the brink of bankruptcy two decades ago.

The 64-year-old was first arrested on 19 November for allegedly underreporting his pay by about 5 billion yen ($45 million USD) between 2011 and 2015.

In addition, he and former Nissan executive Greg Kelly, who was released on bail on Christmas Day, are alleged to have continued to underreport Ghosn’s income through to March this year, increasing the overall underreported amount by 4.2bn yen.

Any hopes Ghosn had of being released before the end of the year were dashed when he was re-arrested days before Christmas, this time for alleged aggravated breach of trust.

Prosecutors claimed Ghosn had used Nissan funds to cover losses of 1.85bn yen ($16.6m) he incurred from derivatives trading in 2008. The extension granted on Monday will allow prosecutors to question Ghosn on this third allegation.

His 10-day detention period for that latest allegation was set to expire on Tuesday.

Ghosn, whose fall from grace has rocked the Japanese car industry, has vowed to clear his name, describing the charges as “absolutely unacceptable” in comments to his lawyer that were reported by Japanese media. “I want to have my position heard and restore my honour in court,” he added.

The Tokyo district court’s decision to extend his detention until 11 January is expected to prompt further criticism of Japan’s criminal justice system, which allows prosecutors to re-arrest suspects several times over different allegations and to question them for up to eight hours a day without a lawyer present.

Under his bail conditions, Kelly is barred from leaving Japan. The American, who denies the charges, is reportedly seeking treatment for a spinal condition at a hospital in Tokyo.

Ghosn, a Frenchman who was born in Brazil and is of Lebanese descent, was once hailed as Nissan’s saviour for masterminding its stunning turnaround in the late 1990s and for forging a three-way alliance with Renault and Mitsubishi Motors.

He was sacked as chairman of Nissan – which has yet to appoint a permanent replacement – but kept on as head of Renault.

The scandal has exposed differences between the French carmaker and its partners in Japan over Ghosn’s alleged conduct that some industry observers say could threaten the alliance.

Shortly before his arrest, Nissan executives were said to be unhappy about his plans to strengthen the firm’s ties with Renault.

Japanese media said Ghosn had admitted signing documents to defer part of his salary until after retirement but said the amount did not need to be declared as it had not yet been definitively fixed.

He is believed to have attempted to defer a portion of his salary to avoid criticism from Nissan staff and shareholders that he was being paid too much.

Nissan has declined to comment on Ghosn and Kelly’s treatment by prosecutors, but claims an internal investigation uncovered “substantial and convincing evidence of misconduct,” by the two men.