EDF Energy is shutting down four of its nuclear power reactors as a precautionary measure after a defect was found in one of them.
The temporary closures of Heysham 1 and Hartlepool account for roughly 10% of the country's nuclear capacity, enough to supply three million homes.
EDF first found a problem at Heysham 1 in Morecambe affecting the boiler spine, a metal tube that supports the weight of boiler tubes around the reactor, last year. The boiler was returned to service early this year on a reduced load, but in June a defect was found and Heysham 1 was shut down.
The firm will shut down the second reactor at Heysham on Wednesday, as well as two reactors at Hartlepool in order to conduct inspections expected to take up to eight weeks.
This will mean the loss of 8.8 million kilowatts of power but National Grid, which runs the UK power network, said there is "a lot of surplus capacity in the system in the summer to handle this outage".
However, the two-month shut down takes the inspection to October when consumer demand begins to pick up.
The power stations at Heysham and Hartlepool were built in 1983 and are set to be decommissioned in 2019. They were originally due to come out of service this year.
EDF Energy's other nuclear plants are at Torness in Dunbar and Hunterston B in West Kilbride, Scotland; Sizewell B in Suffolk; Hinkley Point B in Somerset, and Dungeness in Kent. The company said the issues found at Heysham and Hartlepool are unique to those sites.
EDF added that the two-month shutdown would reduce the firm's annual nuclear power generation by 3.2% to 61 terawatt hours during the year.
Centrica, the owner of British Gas which has a 20% stake in EDF Energy's existing nuclear operations, said the reduction in output from the affected nuclear power stations would reduce the company's 2014 earnings by roughly 0.3p per share.
EDF Energy is leading a consortium which plans to build the new £16 billion Hinkley Point C nuclear plant in Somerset, due to be completed in 2024. The project will create 25,000 jobs during construction as well as 900 permanent jobs during its 60-year operation.