Phoenix Energy has agreed to sell its gas supply business to rival power firm Airtricity.
Its work involves the buying of gas and billing customers for its use.
The growth in the supply network prompted SSE plc (Airtricity), one of the largest energy suppliers in the British Isles, to extend its activities by buying Phoenix’s gas supply arm.
Peter Dixon, group chief executive at Phoenix Energy Holdings, said: “Phoenix Natural Gas, our core business, is responsible for operating and extending our gas distribution network as well as developing and supporting the wider local natural gas industry.
“We have made no secret of the fact that our long-term ambition is to grow our distribution business.”
This has been a separate stand-alone operation within the Phoenix Group for several years.
Phoenix will continue to focus on building Phoenix Natural Gas, which operates the Greater Belfast gas network and is responsible for making gas available to around half the population of Northern Ireland, a company statement said.
Phoenix remains one of the largest private sector investors in Northern Ireland, connecting more than 8,000 homes and businesses to its natural gas network every year. The sale is subject to approval by the Irish Competition Authority.
Phoenix established the natural gas industry in Northern Ireland in 1996 and there are now more than 300,000 homes and businesses using it.
The scale of the gas network has attracted three other suppliers to the Greater Belfast area.
Mr Dixon added: “This growth has now prompted SSE plc (Airtricity), one of the biggest energy suppliers in the UK and Ireland, to extend its current activities in Northern Ireland and the Republic through the acquisition of Phoenix’s gas supply businesses.”