Prime Minister Fumio Kishida is facing yet another setback after handing a key government post to his oldest son, with the opposition bloc lambasting the appointment as nepotism.

The Kishida administration has already been shaken by several controversies, such as his Liberal Democratic Party's ties with the dubious Unification Church and a contentious state funeral for former Prime Minister Shinzo Abe, who was fatally shot in July.

On Tuesday, the government said Kishida tapped his son Shotaro, 31, as an executive secretary to the prime minister, triggering a swift backlash from other parties, including the main opposition Constitutional Democratic Party of Japan.