AUGUSTA — Utility regulators have approved a $9 million rate increase for the largest natural gas provider in southern Maine.

The Maine Public Utilities Commission said today the increase in the natural gas distribution rates for Northern Utilities Inc., or Unitil, applies to all customer classes.

For an average residential natural gas heating customer, the effect of the increase is about $20.50 per bill during the winter months. Overall increases for Unitil’s commercial and industrial customer classes range from 5 percent to 15 percent.

PUC Chairman Thomas Welch says the increases are needed to bring Unitil’s revenues in line with its cost of providing safe and adequate service.

Unitil serves about 26,000 customers in the Portland area.