Shares in Toshiba fell more than 19% today, clocking a third day of heavy losses after the Japanese tech-to-nuclear conglomerate said earlier this week it faced a potential multi-billion dollar writedown. 

Bond yields have also surged and the cost of insuring against its debt have soared. 

Earlier this week, Toshiba said cost overruns at a US nuclear business it bought from Chicago Bridge & Iron last year, CB&I Stone & Webster, meant it could face "several billion dollars" in charges, acknowledging a bruising overpayment. 

Ratings agencies were quick to respond. 

Moody's last night became the second agency to downgrade Toshiba's rating, pushing it deeper into "junk", or non-investment grade territory, with a Caa1 rating, from B3. 

Moody's said the downgrade also reflected "mounting concerns" over corporate governance, especially in relation to due diligence for acquisitions. 

It said the ratings are under review for a further downgrade because of "potential for a further deterioration in Toshiba's operating and financial performance". 

Since Tuesday's warning, Toshiba's share drop has wiped about $6.5 billion off of its market value.