The costs incurred by energy suppliers in the UK hit their lowest point for five years towards the end of 2015, according to market information firm ICIS.

According to ICIS' Power Index, the cost of wholesale energy fell to £36.76 per megawatt hour. ICIS said the milder winter weather, and continuing fall in commodity prices contributing to the fall in wholesale prices. Electricity prices dropped 23% in 2015, while gas fell 34%.

On Thursday, oil prices fell briefly below the $30 per barrel mark. Having dropped from $110 in the summer of 2014, they have now fallen 20% since the start of 2016, alone. ICIS also said oversupply of gas contributed to the decline of its wholesale price.

"More gas from around the world in the form of liquefied natural gas (LNG) is expected to come to the UK, as more production capacity comes on-stream in 2016," ICIS said.

And news of suppliers having their costs reduced has raised questions over why consumers are not receiving the same benefit. British Gas cut its prices by 5% in August, but none of the remaining members of the 'big six' energy companies cut their prices.

Which? executive director Richard Lloyd said: "It's extremely disappointing millions of us are still paying way over the odds for our energy. Consumers will rightly ask why their bills haven't been cut dramatically when wholesale costs have dropped.

"The government needs to protect vulnerable customers from being ripped off and make people feel confident about switching supplier."