Shares in British Energy closed down 8% after problems uncovered by a routine inspection forced it to take four nuclear reactors out of service.
Hartlepool Reactor 1 was scheduled to return to service in early November, but will be delayed after an inspection of its boiler closure units.
British Energy also decided to close two sister units in Hartlepool and Heysham as a result of the inspection.
And the return to service of another reactor in Heysham will be delayed.The boiler inspection revealed a problem related to wire winding.
British Energy shares had fallen as much as 10% before recovering slightly, but the firm was the biggest loser in the FTSE 100 index of the UK's top shares.
The design of the boiler closure units is unique to the Hartlepool and Heysham power stations, it said.
British Energy saw its profits fall for the first quarter of 2007, following stoppages at its Hinkley Point and Hunterston stations.
Interest in nuclear energy has gained ground, with the UK government examining whether to build new plants.