PORTLAND, Maine — The Houston-based Spectra Energy announced Tuesday that Northeast Utilities, New England’s largest electric utility, plans to be a co-investor in a $3 billion effort to expand two major natural gas pipelines in the region, one of which runs through Maine.

Spectra’s pipeline expansion proposal was made public to a regional group coordinating energy policy in June, but the investment partnership with Northeast Utilities is new.

“I think it certainly signals the potential for a partnership that would really bring not only the resources but the experience to address New England’s natural gas capacity problems,” said Patrick Woodcock, director of Maine’s Governor’s Energy Office.

Marylee Hanley, a spokeswoman with Spectra, said the pipeline project, dubbed its Northeast Access project, could be online as soon as 2018 and the companies are now seeking out interest in more pipeline capacity from local distribution companies, including Bangor Gas Co.

“We expect that some of the capacity will be bought by local distribution companies,” Hanley said. “And given the scale [of the project], we can proceed only with those commitments.”

The company plans to take those commitments to the Federal Energy Regulatory Commission to demonstrate regional demand for the expansion as it seeks the required federal approvals.

Hanley said that the project’s timeline does not depend on a regional effort for the states to step in and buy natural gas pipeline capacity, a matter the Maine Public Utilities Commission is considering after passage of the sweeping Omnibus Energy Bill in the Legislature last year. The PUC is considering whether and to what extent it should use the authority to purchase up to $1.5 billion in pipeline capacity over 20 years.

Regulators would have paid for that capacity through a fee on electric bills, with the idea being that natural gas pipeline expansion stands to improve the reliability of the electric grid and lower electricity costs. Part of the PUC’s consideration is what impact incremental additions of natural gas capacity to the region will have on electricity prices.

The issue is politically charged. Republican Gov. Paul LePage last month criticized Democratic Massachusetts Gov. Deval Patrick for stepping away from the regional planning process after the Massachusetts Legislature rebuffed bills that would have called for boosting imports of Canadian hydropower.

Not having Massachusetts in the partnership was a blow to the regional effort as the Bay State stood to be the region’s largest customer for working out long-term hydropower or natural gas capacity contracts.

But Spectra’s plans would move that capacity expansion effort ahead in the private sector alone. Hanley said the estimated $3 billion investment to expand its Algonquin and Maritimes & Northeast pipelines will come through a 50-50 equity investment from the companies.

In addition to increasing the capacity of gas available in Maine, Hanley said that it would diversify the sources of that gas by connecting the Algonquin and Maritimes & Northeast systems, giving Maine more access to gas from the vast reserves opened by the controversial drilling method dubbed fracking and other gas imported through the Gulf of Mexico.

As a result of New England’s pipeline constraints, prices for natural gas remain higher than in other regions that have more access to the surge in supply. That price difference is measured at major hubs around the country and policymakers in the Northeast have sought to bring down the difference in those prices across regions.

Spectra said in June that a separate expansion on its Algonquin pipeline would contribute to a 35 percent reduction in that regional difference, meaning New England would spend about $655 million less annually on natural gas.

Shortages of natural gas during the cold snap last winter has prompted concern from the region’s electric grid operator, ISO New England, because the fuel powers more than half of the New England’s power plants. As home heating uses get first dibs on natural gas coming to the region, high heating demand last winter pinched the amount of gas available for electricity generation, causing wholesale power prices to spike.

The day-ahead market for wholesale power in the region is set by the price of the most expensive fuel to fill out the projected power need for the following day.