All 87,0000 customers will be hit with the hike, which is set to add around £70 per year to householders' bills.
Firmus said: "For customers in the Ten Towns area, the typical household bill will rise by £6.25 per month, equivalent to £1.44 per week.
"The typical household bill in the Greater Belfast area will increase £5.47 per month, equivalent to £1.26 per week."
Michael Scott, Managing Director of firmus energy, said the increase is "unavoidable".
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"Firmus energy continually monitors and reviews its tariffs to ensure customers are getting the best prices possible," he said.
"Our pricing team has engaged extensively with the Utility Regulator, in consultation with the Department for the Economy and the Consumer Council for Northern Ireland in relation to our regulated tariff in the Ten Towns licensed area."
Jenny Pyper, the Utility Regulator’s Chief Executive, said: "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.
"This increase will leave the firmus energy domestic tariff for the Ten Towns area 6.2% lower than the Bord Gais average standard tariff in the Republic of Ireland and higher than the average standard gas tariff in Great Britain."
(LM)
