Britain's Big Six gas and electricity firms are facing fresh pressure to cut tariffs as regulator Ofgem said tumbling wholesale energy prices meant they could afford to lower household energy bills.

Latest figures from the regulator suggest the firms are on course to make £102 profit per household over the coming year compared with £48 in 2013, assuming normal weather and no change in tariffs.

The regulator wrote to the big companies -- British/Scottish Gas, SSE, ScottishPower, E.on, EDF and npower - in the summer asking to explain how they were going to pass on falling wholesale costs to their customers.

But Ofgem noted none of the companies decided to win customers from rivals by reducing tariffs.

Rachel Fletcher, senior partner for the regulator's markets division, said the figures from its Supply Market Indicator (SMI) "might be suggesting there is room for price cuts or that you wouldn't be expecting prices to go up".

She added: "As regulator, it is not for us to tell companies how to set their prices. We are relying on the market to do that.

"But the thing that has concerned us is in a situation where we have got wholesale costs coming down, you would expect some companies to be saying here is an opportunity to gain market share by reducing our prices.

"We haven't seen that. That raises questions for us about the extent to which there is competition between energy companies."

The remarks come with a full-scale competition probe into the energy market under way, which could ultimately see some of the larger suppliers broken up.

Ms Fletcher said Ofgem's decision to refer the sector to the Competition and Markets Authority was the biggest it had made.

"We have pulled it out of the box and used it. That is the strongest thing any regulator can do when concerned about the market.

"Quite frankly we are concerned the market isn't working in consumers' interests."

Ms Fletcher acknowledged suppliers faced uncertainty over wholesale costs as a harsh winter and the Ukraine crisis could push up prices but suggested these would be limited with firms buying much of their energy up to two years in advance.

Ofgem encourages more customers to switch suppliers, saying those who have been with the same company for two years are likely to save £200 a year.

The regulator's SMI figure - for profits likely to be made by a typical supplier over the coming 12 months, all things being equal -has been criticised as unrealistic by the industry.

Ofgem insists SMI is not a forecast. Its methodology would have suggested £59 profit per household for 2013 whereas the actual figure was £48, a fall on the previous year.

But it pointed to a low reading of £15 profit per customer in the middle of last year which suggested suppliers would need to raise their prices. They did that autumn.

The regulator also published details of how much the Big Six suppliers had profited over 2013.

Earnings from household supply were £1.13 billion, lower than £1.19bn the year before but still substantially above the figure of £681 million in 2011 and several times more than the £221m earned in 2009.

Profit margins in 2013 were 3.9%, with companies making £48 out of an average bill of £1,225 compared to £53 out of an average of £1,174 in 2012, or 4.5%. But margins varied between suppliers.

British Gas earned the highest revenues among the Big Six, with 42% of the gas domestic supply market and 89% of the profits. Its profit margin was 8.9%, twice that of the second largest for gas, SSE.