Shares in British Energy fell as much as 8% after the company failed to say when four of its nuclear reactors would return to service.
Last month, the firm took two reactors at Hartlepool and two at Heysham out of service following a routine inspection.
In an update, British Energy said it had discovered wire corrosion at Heysham 1 Reactor 1.
The problem was a similar one to that found at the Hartlepool Reactor 1 plant which led to the shutdown in October.
British Energy said inspections of Heysham 1 Reactor 2 had started, while inspections would begin at Hartlepool Reactor 2 shortly.
The company said the problem uncovered was a "complex issue" and added that "a timetable for the return to service of these units can only be formed when inspections and a full assessment of the situation have been completed".
British Energy's shares closed down 7% at 514.5 pence.
British Energy Group is the UK's largest producer of electricity. It operates eight nuclear power stations and one coal-fired power station.