Prosecutors must swiftly accept Thursday's Shizuoka District Court decision to reopen a high-profile 1966 murder case and get to the truth behind the conviction of former professional boxer Iwao Hakamada, the Japan branch of Amnesty International said after the ruling.

After nearly five decades in which Hakamada, now 78, has been in solitary confinement, no further attempts to delay the retrial are tolerable, the group said in a written statement, adding that his physical and mental health are in decline.

The group described Hakamada's death sentence, which was finalized by the Supreme Court in 1980, as fraught with allegations that he was coerced and tortured into confessing. It also noted that it wasn't until after 2008 that prosecutors disclosed more than 600 pieces of new evidence, some of which undermined the veracity of their earlier evidence. Amnesty said the case represents an attempt to "cover up the truth" and blasted it as an act of "extreme injustice."