The Karachi Electric Supply Co (KESC) said it has obtained a fatwa, or decree, from 12 top Islamic scholars, who declared the theft of power a sin.
"It is astonishing and disturbing to find that certain segments of our society do not even consider theft of electricity 'theft', let alone immoral or illegal," said KESC spokeswoman Ayesha Eirabie.
Typically, people steal electricity by hooking up a wire to overhead electricity cables, siphoning off power without paying for it.
Another way people steal power is to slow down their electricity meters.
The KESC, in which Dubai-based Abraaj Capital has a controlling stake, supplies electricity to Karachi, Pakistan's biggest city with more than 16 million people.
Eirabie said electricity theft was causing the KESC losses of billions of rupees (millions of dollars) a year. Most of the people who steal electricity could afford to pay for it but choose not to, she said.
"It is very important for such people to know that the theft of electricity is illegal, immoral and not acceptable as is any other form of theft," she said.
According to the decree, a copy of which was seen by Reuters, the illicit use of any commodity is a sin.
And as in this case the organization producing electricity represented many people, its use without permission and pay made it an even bigger sin, the clerics said.
"Legal action against such people committing electricity theft is fair," they said.
Karachi, like the rest of Pakistan, has faced chronic electricity supply problems. Residents frustrated by blackouts often vent their anger by blocking roads, burning tires and throwing stones at police.