A former boxer who chopped off the penis of his wife's lover with a pair of garden shears before flushing it down the toilet has been jailed.

Ikki Kotsugai, 25, carried out the 'vicious' attack in August 2015 when he burst into the office of a lawyer who had an affair with his wife.

Kotsugai, who was studying law at Keio University Law School in Minato, Tokyo, became enraged after mistakenly believing his wife had been forced to sleep with her boss at a legal firm, when in reality the affair was consensual, the judge said at Tokyo District Court.

Ikki Kotsugai, 25, carried out the 'vicious' attack in August 2015 when he burst into the office of a lawyer who had an affair with his wife.

Dismembered: With his victim incapacitated having repeatedly punched him in the face, the boxer hacked away at the base of his penis with the 20cm garden shears while his wife watched on

Kotsugai's wife had falsely told him that she was forced to have sex with the lawyer, according to the Tokyo Reporter.

He was sentenced to four years and six months on Tuesday by presiding judge Kazunori Karei.

'An irreparable injury has been sustained,' Karei told the court, according to Jiji, calling the crime 'extremely dangerous and vicious'.

Kotsugai has since apologised to the victim, who also apologised for having the affair with his wife, the Sankei reported.

On August 13 last year, Kotsugai visited his wife's law firm in Toranomon, Tokyo, and confronted the victim, who was 42 at the time.

After exchanging words, the former boxer repeatedly punched his wife's lover in the face until he was barely conscious on the office floor.

With his victim incapacitated, he unzipped his trousers and hacked away at the base of his penis with the 20cm garden shears while his wife watched on.

Having dismembered his victim, he ran to the office's communal bathroom grasping the penis and flushed it down the toilet, according to Japan Crush.

Having cut off his victim's penis, the former boxer ran to the Toranomon law firm's communal bathroom grasping the penis and flushed it down the toilet

When Akasaka Police arrived at the scene, the victim was gasping and covered in blood.

Kosugai was arrested at the scene and the missing penis was never found, despite a search in the sewers, according to Japan Crush.