Toshiba shares fell more than 20 per cent on Thursday, the third straight day of heavy losses.
The Japanese industrial giant has now had nearly half its market value wiped off since Monday, the BBC reported.
It comes after the firm warned that its US nuclear business may be worth less than previously thought.
Shares were down 25 per cent at one stage in Tokyo, having lost 20 per cent and 12 per cent in previous sessions.
Toshiba said the possible heavy one-off loss was linked to a deal done by a US subsidiary, Westinghouse Electric, the BBC noted.
Toshiba President Satoshi Tsunakawa apologised for “causing concern”.
Rating agencies Moody’s and R&I have both downgraded Toshiba’s credit rating, which will make it more expensive for the company to borrow. R&I warned that it could issue further downgrades.
While the share price slump is a blow for investors, 2016 had still been a pretty good year, with the stock rising more than 77 per cent before this week’s falls.