DUAL-fuel customers of the big energy firms have missed big savings by not switching suppliers, early evidence from a competition inquiry suggests.

The Competition and Markets Authority (CMA) has been investigating the energy market since last summer.

It says that from 2012 to 2014, more than 95% of dual-fuel customers of the big firms would have have saved money by switching tariffs or suppliers.

The savings they missed ranged from £158 to £234 a year per customer.

The investigation by the Competition and Markets Authority (CMA) was formally launched last July in response to an earlier referral from the energy regulator Ofgem.

Ofgem had been concerned because of widespread disquiet at the dominance of the industry by just six big operators.

Currently the “big six” energy firms – SSE, Scottish Power, Centrica, RWE Npower, E.On and EDF Energy – together account for about 92% of the UK’s energy supply market.

The update summarises the CMA’s thinking so far and point to issues for further investigation.

It highlights the plight of millions of “sticky” customers who were inherited by the big suppliers following the liberalisation of the energy market.

Millions of these customers, who are on standard variable tariffs, rarely switch and are therefore missing the opportunity to save money.

The report says that 40% to 50% of customers have been with a supplier for more than 10 years. For one supplier the figure is as much as 70%.

The report says there is some evidence that the six large energy firms have “unilateral market power” over these standard variable tariff customers.

It notes that the firms have consistently charged higher prices for these customers compared with those on non-standard tariffs which “provides some support to the view that these suppliers can segment the market and price discriminate”.

But it adds that it is not clear whether this is a deliberate ploy of the big suppliers.

The CMA has not formed a view on the profits of the big six suppliers, saying that “we are continuing to look at whether overall profit in energy retail has exceeded an appropriate benchmark”.

The CMA will also investigate the idea that recent decisions by the energy regulator Ofgem have done more harm than good by reducing the number of tariffs available to consumers.

The CMA’s thinking is more advanced on issues around power generation.

It does not think so far that firms have been making excessive profits from generating electricity. It also says that the evidence it has reviewed does not suggest that wholesale prices are opaque or that the market lacks liquidity.

It also sees no significant issues with the big energy firms owning both power generation and supply arms.

A break up of the big six is therefore unlikely, the BBC understands.