TOKYO (Reuters) - Japan's SoftBank Group Corp 9984.T said Tadashi Yanai, founder and CEO of Uniqlo parent Fast Retailing 9983.T, will resign as independent board member at the end of the month after 18 years on the job to focus on his fashion business.

FILE PHOTO: Tadashi Yanai, chairman and chief executive of Fast Retailing Co, attends a joint news conference with United Nations High Commissioner for Refugees Antonio Guterres (not pictured) in Tokyo, Japan, November 25, 2015. REUTERS/Toru Hanai

A longtime ally and sometime critic of SoftBank founder and CEO Masayoshi Son, the billionaire is one of only three external members of a board filled with SoftBank executives and heads of its portfolio companies.

The resignation comes at a time when SoftBank is battling with the fallout from the failed IPO of WeWork, with Son saying he misjudged co-founder Adam Neumann’s character and pledging to strengthen corporate governance at the group’s investments.

However, experts are critical of SoftBank’s governance, saying it has few truly independent voices that can question Son’s judgement.

“They have low governance standards,” said Nicholas Benes of The Board Director Training Institute of Japan, a non-profit focused on corporate governance training.

“If they don’t require higher standards of themselves, it might be hard to require them of investee companies,” he said in an interview last month.

Son in November defended the board’s rigour after reporting the group’s first quarterly loss in 14 years, saying that Yanai was among the board members who excoriated him for the WeWork investment.

“Almost all the board members gave me a hard time. I ended up being very exhausted,” he said.

Yanai, Japan’s richest man, along with Son, are among a handful of Japanese founder-CEOs who are also household names.

Yanai was known for being willing to voice his dissent at some of Son’s decisions.

His successor has not been decided, a SoftBank spokeswoman said.

SoftBank lost one of its most outspoken voices when another outside director, Shigenobu Nagamori, founder and CEO of Nidec Corp 6594.T, stepped down two years ago.

The other independent directors currently on SoftBank's board are Masami Iijima, chairman of trading house Mitsui & Co 8031.T, and professor Yutaka Matsuo from the University of Tokyo, a highly renowned artificial intelligence expert with little corporate experience.

Both Yanai and Son have made and then shelved plans to hand over the reins of their companies in the past, with 70-year-old Yanai previously saying he would retire at 65.

Yanai has said he does not want either of his two sons to take over as CEO, but both were promoted and joined the ranks of company directors last year.

Possible successors named by media have included finance chief Takeshi Okazaki and Pan Ning, the head of Uniqlo’s China operations, as well as Maki Akaida, head of Uniqlo’s Japan operations.