EDF Energy has been forced to halt both reactors at its 1,280 megawatt (MW) Torness nuclear plant near Edinburgh after a rising tide of seaweed threatened to clog its cooling system.
Britain currently has nine reactors offline with a combined capacity of more than 5,000 MW, or around half of the country's total nuclear capacity.
"Around 11.30 on Thursday May 23, Unit 2 at Torness power station came offline due to increased seaweed levels as a result of the severe weather and sea conditions in the area," EDF said.
"This was followed by a decision to take Unit 1 offline just after 3 am today ...when it was clear that the seaweed levels weren't reducing."
According a regulatory update on EDF's website, the 640-MW reactor 1 will be off line until June 7 while 640-MW reactor 2 is not expected to supply the grid until May 28.
EDF said staff are trained to deal with high seaweed levels resulting from weather conditions in the Forth Estuary.
The Torness plant was forced to shut down in 2011 after large numbers of jellyfish were found in the sea water entering the plant.
Data from power company Exelon showed that nuclear generation was just 10pc of Britain's total power output on Friday.
Day-ahead British baseload electricity was up £1.25 to £52.67 per megawatt-hour.