Natural gas supplier Firmus has announced a price hike of 12% for its more than 85,000 customers in Northern Ireland.
The tariff increase will take effect next month.
Firmus has around 50,000 domestic and business customers in Greater Belfast area and more than 38,000 customers in 10 towns.
Announcing the price rise, Firmus Energy managing director Michael Scott said: "We know that price stability is important for our customers and regrettably this increase is unavoidable".
He said Firmus continually monitored and reviewed its tariffs to ensure customers were getting the best prices possible.
The typical household bill in the Greater Belfast area will increase £5.47 per month, while customers in the 10 towns will see their bills rise by £6.25 per month.
The towns include Coleraine, Craigavon, Limavady, Londonderry and Newry, among others.
The Consumer Council said the tariff increase was a further blow for consumers following recent increase by the three largest electricity suppliers in Northern Ireland.
Head of energy policy Richard Williams said: "This is unwelcome news for Firmus Energy customers, following a 6.8% increase in the 10 Towns area and 8.8% increase in Greater Belfast earlier this year."
Mr Williams said he would encourage customers in the Greater Belfast area to shop around and get the best possible deal.
However, customers in the 10 towns do not have that option as Firmus Energy is the sole supplier available to them.
Mr Williams said the council would like to see other gas licence holders enter the market so that those consumers can benefit from competition.
Utility Regulator chief executive Jenny Pyper said: "We do understand that price increases are always unwelcome and so we act on behalf of consumers to ensure any changes to the tariff are fully justified."
She added: "The main reason for this increase is due to the significant rise in global gas costs, which make up the largest portion of customer bills, since the tariff was last set earlier this year in April.
"A small rise in the cost of transmission charges has also contributed to the tariff increase."