Shares in Japanese engineering conglomerate Toshiba shares fell 20% after it warned it would make writedowns at its US nuclear business amounting to "several billion dollars".

The cut in value relates to Westinghouse, the nuclear construction and services business, it bought from Chicago Bridge & Iron for $229m (£187m) last year. There is now a dispute over the costs of the deal and the value of the assets it took on.

The fall comes after a 12% slide in the stock price yesterday (Tuesday December 27), following early reports that Toshiba would impair the value of its nuclear business.

The slump has wiped almost $5bn off the group's value in two days and prompted a credit rating downgrade.

Ratings agency Standard & Poor's downgraded Toshiba, already in junk territory, to B- from B, with a "negative" outlook.

The stock closed on Wednesday at ¥312 a share (£255) in Tokyo, its biggest one-day fall since 1974.

Toshiba chief executive Satoshi Tsunakawa told a press conference that the semiconductor-to-nuclear group was "mulling ways to raise additional capital".

The writedown deals a heavy blow to the group, which is trying to rebuild investor confidence after a 2015 accounting scandal that led to the firm admitting it inflated net profits by $1.3bn over seven years.