"We have encountered some teething problems during the process of raising load," chairman Adrian Montague said at the group's annual general meeting.
"Delivering increased power levels will not be straightforward," he continued. "I have to say that returning to 100 per cent power is unlikely."
Group shares fell by 1.1 per cent following the announcement, with it now valued at around £5.4 billion.
British Energy has a total of eight nuclear plants and one coal-fired station and is capable of producing around 20 per cent of the total electricity needed in the UK. Its ageing stations have suffered from a series of faults in recent years, however.
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