The electricity generator is planning to extend the plants' lives from 25 to 30 years following "detailed engineering reviews" by the company and outside consultants.
British Energy, which owns eight nuclear power stations and one coal-fired plant, revealed the move ahead of a statement showing a pre-tax loss of £56m for the half year to the end of September.
The firm, which stands to gain £29m a year from keeping the plants open, blamed falling electricity prices for much of the loss, which compared to a £55m profit for the same period in 1999.
Boiler leak
The firm also admitted that a boiler leak at the Hunterston nuclear power station, and design restrictions which hindered the uranium feed into the Torness plant, saw electricity output fall below forecasts.
"There were absolutely no safety implications," British Energy spokesman Bob Fenton told B News Online.
One of two reactors at Dungeness B, in Kent, is "in the final stages" of returning into service following repairs.
The firm said earnings for the full year were set to fall significantly below the pre-tax profit of £241m.
'Consolidation'
But the company, which has launched a productivity drive at its power stations, had predicted the year would be one of "consolidation", it said.
"Earlier this year I said that I expected this year to be challenging," chairman Sir John Robb said. "This has proved to be the case."
The leasing of a Canadian plant, which will see British Energy's generation portfolio rise 80% in two years, and productivity improvements leave the company "confident" of returning to profit, Wednesday's results statement said.
British Energy, privatised in 1996, was formed through the merger of Nuclear Electric and Scottish Nuclear 10 years ago.
It does not operate the ageing Magnox nuclear reactors, which are owned by BNFL.
British Energy shares closed 12.5p up at 175p on Wednesday in London.