Questar Pipeline Co. is proposing an increase in the rate it charges Mountain Fuel Supply Co. for natural gas.

Questar Pipeline and Mountain Fuel Supply are both subsidiaries of Salt Lake-based Questar Corp. Under an inter-company agreement, Questar Pipeline delivers gas from the well head to Mountain Fuel, which then distributes it to residential, business and industrial customers in Utah, Wyoming and Idaho.Questar says the increase from $3.31 per decatherm to $3.45 per decatherm results from a quarterly rate adjustment required by the Federal Energy Regulatory Commission. If approved, the new rate will go into effect June 1.

While this increase will not be felt immediately by Mountain Fuel customers, it will affect the company's semi-annual rate adjustment proposal that goes to the Utah Public Service Commission.

Mountain Fuel recently received interim approval for an $11.9 million pass-through rate increase. A formal hearing on that increase is scheduled April 3.

Questar officials say the increase is needed to cover a relatively high cost-of-gas period due to low sales levels and correspondingly high percentages of high-cost gas in Questar Pipeline's gas supply mix.

The company expects the adjustment to be in effect through June 1, 1992.