Up to 42,000 homes have had their power cut off again after almost all were reconnected following floods in Lancashire.
Supplies were lost in Carnforth, Lancaster and Morecambe at 16:00 GMT.
Only 300 properties out of 55,000 had needed reconnecting after floodwater breached defences at Lancaster's main substation on Saturday.
Electricity North West apologised and said "unforeseen" flood damage at the site caused a "significant" fault.
Many street lights and traffic lights were also off because of the power cut in Lancaster during the evening rush-hour.
Lancashire Police tweeted: "There may be issues with traffic lights due to the power cut so if you are travelling by car to check on someone please drive carefully."
The force also urged pedestrians to "make sure" they can be seen.
Electricity North West said an assessment of the damage has "resulted in further loss of supply at Lancaster" but the number has not been revealed.
Engineers have plans in place to begin the restoration of supplies from approximately 08:00 on Tuesday, it said.
Power cuts have been also been reported at Carnforth and Lancaster, Morecambe train stations.
Mark Williamson, operations director, said: "Unfortunately power has been lost to customers in Lancaster and surrounding areas due to unforeseen flood damage at our substation.
"We plan to begin to restore supplies from the substation tomorrow morning."
The firm has secured extra generators from across the UK and they will be connected throughout the night.
Queen Victoria Hospital in Morecambe has announced its outpatient clinics will be closed on Tuesday due to the power outage.
While Royal Lancaster Infirmary will be offering limited surgeries and outpatient appointments.
Engineers have been working "around the clock" to restore supplies after the initial outage, which left schools in Lancaster and Morecambe closed.
Power had been restored to homes using generators and residents were warned of a "very short outage" once they were back on the main supply.
Up to 19,000 residents in Lancaster are still being supplied by those generators.
The Met Office has predicted further heavy rain in the Lancaster area from about 21:00 until the early hours on Tuesday
Further south there was a partial collapse of flood defences on the River Wyre at St Michael's South, the Environment Agency said.