A South Korean man is suing former German chancellor Gerhard Schroeder for allegedly having an affair with his ex-wife, reports say.
Mr Schroeder is set to marry the woman, South Korean translator Kim So-yeon, who will be his fifth wife.
But Yonhap news agency says her former husband believes they had a liaison while she was still married to him.
In South Korea, extramarital affairs were a criminal offence until 2015, and remain grounds for civil litigation.
The husband is seeking 100m won (£68,000; $94,000) for "unbearable mental distress", Yonhap adds.
In a filing to the Seoul Family Court, Ms Kim's unnamed former husband reportedly says Mr Schroeder and Ms Kim began dating while both were still married.
"Our marriage eventually fell apart, and the accused should be held responsible for his action," the filing reportedly says.
Mr Schroeder, currently serving as chairman of Russian oil company Rosneft and the Nord Stream pipeline, announced his engagement to Ms Kim in January.
A translator and Seoul's representative on North Rhine-Westphalia's economic development agency, Ms Kim is 26 years younger than her future husband.
She acted as a translator for Mr Schroeder at a management meeting two years previously, and the pair became close.
His last marriage to journalist and politician Doris Kopf fell apart in 2016.
In a Facebook post (in German), Ms Kopf said the relationship with Ms Kim was one of the reasons for their break up.
Mr Schroeder, the Social Democratic chancellor of Germany from 1998 to 2005, is well known for his colourful private life.
He has earned the nicknames Audi Man and The Lord of the Rings for his numerous marriages.
This court case is the latest controversy to hit the 74 year old.
His appointment as chairman of Rosneft in 2017 caused widespread criticism, deepening concerns about his close relationship with Russian president Vladimir Putin.
Norbert Roettgen of Germany's Christian Democratic Union has even accused the former chancellor of trying to increase German energy dependence on Russia.