A decades-old Supreme Court ruling, interpreted as allowing Japan to defend allies as part of self-defense, may be used to justify Prime Minister Shinzo Abe's move to lift a self-imposed ban on exercising the right to collective self-defense, government sources said Tuesday.

One of the options is to cite a supplementary opinion by former Chief Justice Kotaro Tanaka, who said in 1959 that Japan had the obligation to defend other countries, and argue that Japan can now exercise the right to collective self-defense, the sources said.

The government is considering including Tanaka's supplementary opinion in its basic stance to be announced before it seeks Cabinet approval to enable Japan to defend allies under armed attack.