Toshiba expects 300 billion-400 billion yen ($2.4 billion-$3.2 billion) in charges related to improper accounting in an expanding probe that is set to force Chief Executive Hisao Tanaka to step down, sources familiar with the matter said on Wednesday.

The Japanese conglomerate has hired a third-party committee to investigate past book-keeping practices which sources say led to profits being overstated by more than 170 billion yen. That's more than triple Toshiba's initial estimate of around 50 billion yen.