The state ordered National Grid not to raise delivery rates for its 980,000 natural-gas customers in Brooklyn until 2015, after finding that the company overcharged those customers by nearly $70 million in 2010 and 2011.

The Public Service Commission took the action after estimating the overcharge was about $2.90 a month — roughly $34 a year — during the two-year period.

Customers’ bills will still fluctuate as the price of natural gas rises or falls, but the portion that reflects the cost of maintaining gas lines will stay flat until Jan. 1, 2015.

The company said the PSC’s move will help ensure “stable customer rates.“