The Japanese industrial giant has now had more than 40% of its value wiped off since 26 December.
It comes after the firm’s chairman apologised and warned that its US nuclear business may be worth less than previously thought.
Shares were down 26% at one stage in Tokyo, but pulled back some of those loses to close 17% lower.
Toshiba stocks had already lost 20% on Wednesday and 12% on Tuesday.
Why are Toshiba shares tanking?
Most people still recognise the name Toshiba for its electrical products, but it is a very diverse conglomerate.
And these latest problems stem from its nuclear services business which brings in about a third of its revenue.
Toshiba said on Wednesday that it faced a possible heavy one-off loss, linked to a deal done by its US subsidiary, Westinghouse Electric.
Westinghouse bought a nuclear construction and services business from Chicago Bridge & Iron in 2015. But assets that took on are likely to be worth less than initially thought – and there is also a dispute about payments that are due.
It has this week also reported “inefficiencies” in the labour force at CB&I, along with other factors driving up costs.
The size of the writedown is not likely to be established until February, but is expected to run to several billion dollars.
Why does a falling share price matter?
Shares have fallen for a reason – investors selling up because of the unease they feel about the position the company is in. That uncertainty saw ratings agencies including Moody’s and S&P cut their ratings on Toshiba’s credit, making it more expensive for the firm to borrow money.
A lower share price also reduces the amount of new funds that can be raised by selling shares. So if it needs to raise funds, that means going to the banks for support or selling off parts of the business. Toshiba recently sold its medical business – which had been doing well – to rival Canon.
And clearly for investors who’ve held on to Toshiba shares, they are now worth markedly less than they were just a few days ago. Longer term though, Toshiba shares had been doing very well in 2016. Until 26 December it was the second biggest gainer on the Nikkei 225 index for the year – adding more than 70%. Recent days though mean those annual gains have been pared to about 5%.
What does this mean for Toshiba’s reputation?
Even if it is a one-off loss rather than ongoing, it seems somebody somewhere got the numbers wrong or did not anticipate the scale of problems at CB&I. And that reflects badly on the firm’s management.
Toshiba is still struggling to recover after it emerged in 2015 that profits had been overstated for seven years, prompting the chief executive to resign.
Toshiba president Satoshi Tsunakawa has this week apologised for “causing concern”.
What are the prospects for Toshiba’s nuclear business?
Toshiba’s nuclear business has not made a profit since 2013.
And while the firm has said the writedowns announced this week were a one-off, nuclear services globally are struggling.
Since the Fukushima disaster in 2011, nuclear energy has been a much harder sell. Some governments have opted to scale back how much they planned to rely on nuclear as an electricity source, or (as in the case of Taiwan) turn away from nuclear energy altogether to focus on renewables.
Big nuclear projects around the world have faced heavy delays, partly caused by a lack of skilled workers needed to meet regulatory standards.
For example in the US, Westinghouse (which Toshiba bought in 2006) is working on two new generation nuclear reactors in Georgia and South Carolina which are running late and over budget.