An embattled key player in Copeland’s proposed new nuclear power station has seen a record rise in its share price.

Shares in Toshiba – which has a 60 per cent stake in NuGen, the company with plans to build a power plant in Moorside, near Sellafield – rose by more than 22 per cent in overnight trading on the Tokyo Stock Exchange.

This is the largest in the company’s history, according to data compiled by the business news service Bloomberg.

The shares closed at 224.7 yen (£1.60) on Tuesday, up from 190 yen (£1.35) at the start of the day.

The company hopes to raise money from its flash memory unit to help cover the 713bn yen (£5bn) loss in its nuclear business, which has fuelled worries over its future involvement in Moorside.

Toshiba said last week that it remained committed to investment in Moorside but would eventually seek to sell its stake to new investors. It also hopes to participate without taking on risk from construction work.

Tuesday’s rise was in sharp contrast to Friday, when the company’s share price fell by 10 per cent after credit rating agency S&P Global described the Japanese giant’s finances as “extremely fragile”.

S&P announced that it may downgrade the credit rating of Toshiba if it receives financial support which includes debt restructuring.

If this happened it would affect the way lenders would be willing to deal with the company.