Some 45,000 homes and businesses have already been cut off after local power substations were swamped by flood waters that have engulfed parts of southern England following torrential rain.
Benn told Parliament on Monday that at least 200,000 more properties could be hit if extra defences rushed into place at a power distribution station at Walham failed to keep back flood waters surrounding it.
"Armed forces personnel were drafted in to help fire service and environment agency staff to erect a kilometre long temporary barrier around the site and to start pumping out 18 inches of flood water behind the barrier," Benn said.
"So far these defences are holding but the water is still rising so it is touch and go."
A spokesman for National Grid, which owns the Walham station, said water levels around the site were rising on Monday afternoon because the tide was coming in up the river Severn, one of several rivers that have burst their banks after Friday's torrential rain.
The UK's Met Office has forecast more rain overnight Monday but expects it to drench southeast England, away from the worst hit flood areas of western England.
