Sizewell B will not generate electricity for three months to enable essential repairs - and EDF will have to submit a "robust safety case" to regulators before it is switched back on.
The nuclear power station - which supplies electricity for 2.5million homes and businesses - has already been offline for six weeks for regular maintenance and refuelling and it was hoped it would be working again next week
However, signs of wear have been found on a thermal sleeve - and the repairs needed will mean keeping the complex offline until August 30.
The problem was anticipated and EDF engineers worked with specialists to assess the issue.
An EDF Energy spokeswoman said: "Sizewell B was brought offline on April 16 for planned refuelling and maintenance work. Our anticipated return to service has been updated to August 30 following additional work required on a thermal sleeve whilst other work on site continues.
"Thermal sleeves are components which are located above the reactor in the pressure vessel head and provide no significant nuclear safety related function.
"In this instance, we have found a worn thermal sleeve and we are now inspecting all other thermal sleeves, as was already planned.
"At no time were there any safety concerns for staff or members of the public.
"This issue is one our industry is aware of and has been subject to extensive assessment at Sizewell B and internationally. Contractor specialist teams are already in place to complete the inspection and repair work whilst the other outage work continues to plan."
A spokesman for the Office for Nuclear Regulation (ONR) said: “We are aware of this issue [relating to the thermal sleeve wear at Sizewell B] and have had extensive technical discussions with EDF as they develop plans to address it. The safety of workers and public was not compromised and a range of safety barriers remain in place.
"Sizewell B is the only Pressurised Water Reactor in the UK so we have incorporated learning and best practice from our international counterparts to inform our regulatory approach.
“We will only allow a nuclear reactor to operate if it is safe to do so. EDF will be required to submit a robust safety case which demonstrates to our satisfaction that this issue has been addressed and that the reactor is safe to operate.”
During the maintenance and refuelling, power station workers are completing 8,000 routine maintenance and inspection tasks and replacing a third of the fuel.
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