PORTLAND – Customers of Northern Utilities, southern Maine’s principal supplier of natural gas, may soon be hit with another rate hike.

Northern Utilities is preparing to ask state regulators to approve an emergency rate increase to cover record-high wholesale prices. The company said it has yet to determine the size of the increase, which could take effect in January.

“We’re still going through the filing and we don’t know what the impact will be,” Carol Churchill, a company spokeswoman, said Thursday.

The last rate hike was approved by the Public Utilities Commission in October and took effect Nov. 1.

Retail natural gas prices are regulated in Maine, and Northern typically files for a cost-of-gas adjustment twice a year. But because wholesale prices have been so volatile, the company has already gone to the PUC twice this year for emergency changes.

The average winter bill for customers who use gas for heat, hot water and cooking is now projected to be $194 a month, according to Northern.

The federal government has warned that gas consumers could pay 50 percent more to heat their homes this winter. Roughly half the homes in the country are heated with gas, although the fuel has played only a minor role in Maine.

That’s slowly changing with the completion of two new interstate pipelines. Northern has a total of 23,000 customers in southern Maine. It has expanded recently to Kittery and is building a connector to Sanford.

A newly formed competitor, Maine Natural Gas, has begun serving Windham and Gorham and is planning to expand to Brunswick. That company markets its gas through monthly or fixed-priced contracts and doesn’t file comparable rate cases with the PUC. It said Thursday that prices are still competitive with home heating oil, which has been selling at a statewide average of $1.55 a gallon.