Shares in British Energy plunged by nearly 9 per cent in early trading as the beleaguered group revealed fresh safety concerns at two of its ageing nuclear power plants.

The group, which generates a fifth of the UK’s electricity, said “an issue with wire winding” had been discovered at Reactor 1 in Hartlepool during a routine inspection.

The setback comes just two months after Bill Coley, the chief executive, insisted that performance at the group was on an “improving trend”.

The latest problem will delay the restart of production at the Hartlepool reactor and British Energy said two more units would have to be taken out of service and checked for the same issue.

It added that the resumption of production of Heysham’s Reactor 2 would also now have to be delayed.

In a statement, the group said: “We are currently assessing our return to service dates for these units.

“In determining these dates, British Energy will consider the potential to integrate any additional inspection work arising from our assessment within planned outages over the balance of the year.”

The shares fell 50p to 529p, wiping more than £500 million from the group’s market value and making British Energy the heaviest faller in the FTSE 100.

Despite sky-high electricity prices, British Energy has suffered falling profits because of a spate of production problems since discovering cracked pipes in boilers at Hinkley Point B and Hunterston B a year ago.

In August, the group revealed that underlying profits in the first quarter were down 12 per cent with output 15 per cent below the same period a year ago.

It added that it may not be able to boost production capacity at Hinkley Point and Hunterston to 70 per cent for at least a year because of temperature restrictions.

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