The wife of former Nissan chair Carlos Ghosn has written to Human Rights Watch to protest against her husband’s detention in Japan.

“My husband’s is a case study in the realities of this draconian system,” Carole Ghosn wrote in a letter to the group’s Tokyo branch.

Mr Ghosn was arrested on 19 November and has been charged with underreporting his income by tens of millions of dollars and breach of trust. The businessman maintains he is innocent.

According to Ms Ghosn, the former Nissan boss is being held in an unheated 75sq-ft cell, being denied medication and has mainly rice and barley to eat. As a result he has lost 3kg in two weeks, she claims.

The prolonged detention of the motor industry’s most prominent executive has put Japanese justice under the spotlight more than ever before. Prosecutors have a conviction rate of more than 99 per cent, while the accused are frequently questioned for weeks or even months without access to a lawyer.

“For hours each day, the prosecutors interrogate him, browbeat him, lecture him and berate him, outside the presence of his attorneys, in an effort to extract a confession,” Ms Ghosn said.

“No human being should be detained under conditions so harsh that their only plausible purpose is to coerce a confession.”