UPDATE 1-Uniqlo founder Yanai resigns as SoftBank board member

* Yanai served on SoftBank board for 18 years

* Board filled with company insiders

* Yanai's successor to board not decided (Adds details of board composition)

TOKYO, Dec 27 (Reuters) - SoftBank Group Corp said on Friday Tadashi Yanai, founder and CEO of Uniqlo parent Fast Retailing, would resign as external board member at the end of the month after 18 years to focus on his fashion business.

An ally 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.

Yanai's successor to the board is not decided, a SoftBank spokeswoman said.

Yanai, who along with Son is one of a handful of Japanese founder-CEOs who are also household names, was known for being willing to voice his dissent at some of Son's decisions.

SoftBank lost one of its most outspoken voices when another outside director, Shigenobu Nagamori, founder and CEO of Nidec Corp, stepped down two years ago. (Reporting by Sam Nussey; Editing by Muralikumar Anantharaman)