GAS prices are likely to fall next month when the Utility Regulator completes a review following global falls in energy costs.

It has been a winter home heating windfall for domestic and business oil customers, with prices plummeting to their lowest for 11 years.

Some householders have been puzzled by the failure of gas to follow suit, but they work in two separate markets, with prices based on the respective international wholesale energy costs.

The oil market follows - to a greater or lesser extent - the crude oil `barrel' price and home heating oil is also unregulated, leading a degree of competition between companies.

However, along with the electricity and water industries, gas is regulated by the Utility Regulator in Northern Ireland.

Earlier this month, it announced a review of the electricity and gas tariffs which is to be completed by mid-February and is likely to reflect falling global oil figures.

A gas bill is around 40 per cent wholesale gas costs while the remaining 60 per cent includes operating and networks costs - taking into account transportation and infrastructure charges to distribute gas from Britain to Northern Ireland.

Oil does not have the same network or infrastructure costs, making the price of gas less volatile than oil and its increases and decreases more gradual.

Firmus energy is the only domestic supplier in the 'ten towns' area of Derry, Limavady, Coleraine (including Portstewart and Bushmills), Ballymoney, Ballymena (Broughshane), Antrim (including Ballyclare and Templepatrick), Craigavon (including Portadown and Lurgan), Banbridge, Newry (Warrenpoint) and Armagh (Tandragee).

In greater Belfast there are two domestic suppliers, SSE Airtricity, which is price regulated, and firmus energy.

"The Utility Regulator directly regulates the gas prices that SSE Airtricity Gas Supply (in greater Belfast) and firmus energy (in the ten towns) can charge to their domestic and small business consumers," a spokeswoman for the regulator said.

"These consumers can be sure that their bills reflect no more than a regulated margin and the actual costs of supplying gas to homes and businesses, and that all reductions in wholesale gas costs are passed onto them. Indeed, since April 2015, SSE Airtricity's prices in the greater Belfast area have fallen by 17 per cent.

"We carry out a formal review on SSE Airtricity Gas Supply and firmus energy's prices bi annually.

"However, we can also initiate a further review at any stage should the wholesale cost of gas change significantly. Due to our regulation, we have complete transparency of the reasons for any price movements.

"Electricity and gas consumers in Northern Ireland are paying amongst the lowest standard tariffs in the UK. We are currently reviewing the regulated energy suppliers' tariffs and expect to make an announcement in mid/end of February."