Yuriko Koike, who is aiming to become Japan’s first female leader, received a boost to her campaign on Friday after the popular former prime minister, Junichiro Koizumi, endorsed her in the forthcoming ruling Liberal Democratic party’s leadership election.

The backing of Mr Koizumi is a boon for Ms Koike, but is not expected to dent the clear lead of Taro Aso, LDP secretary-general.