Toshiba Corp's top investor has called for an investigation of a botched meeting in July at which it says several shareholders were deprived of their vote, according to a letter to the conglomerate's board seen by Reuters.

What began as a problem at Toshiba has widened into a larger scandal for corporate Japan, with the bank hired to count the votes, Sumitomo Mitsui Trust Bank, and Mizuho Trust & Banking Co. revealing details of counting errors at more than 1,300 firms.

Singapore-based Effissimo Capital Management, which owns around 10 percent of Toshiba, is the latest investor to join the outcry over the meeting, which has drawn attention to concerns about governance and treatment of foreign shareholders.