AS the time to switch on the heating again looms, energy groups are urging consumers to cut their winter bills by looking beyond the big suppliers to smaller competitors.

A report by Citigroup has predicted that the average duel fuel bill will rise by 20% over the next five years, by which time the "Big Six" energy firms may have lost one-quarter of their customers to the lesser-known names. British Gas, SSE, EDF Energy, E.On, ScottishPower and Npower currently control about 92% of the household supply market, but this is likely to fall to less than 70% by the end of the decade, the report said.

Ann Robinson, director of consumer policy at comparison website uSwitch.com, said: "This report is a wake-up call to the 'Big Six'. Consumers are voting with their feet and choosing suppliers which offer competitive prices and good customer service.

"Consumers should take control of their bills now and check to see if they're on the most competitive tariff. Most people who switch save, on average, £200 a year."

There are now 18 smaller suppliers. The biggest of these, First Utility, has 550,000 customers. Ovo has 350,000 and Ecotricity, which relies mainly on wind power, has 135,000.

According to comparison website energyhelpline.com the best deal currently on offer - from new market entrant Extra Energy, whose German sister firm powers around one million homes and businesses - would save the typical household £342 a year against watchdog Ofgem's benchmark average bill of £1330. Extra Energy's fixed-price deal until November 2015 has an average cost of £988.

Eight of the top 10 deals listed by energyhelpline are from smaller players, and even the 15th cheapest deal would save an average £259 a year.

Many switches are prompted by poor customer service, regulator Ofgem has suggested. In the first half of this year alone, 1.7 million complaints were made about the "Big Six" firms. Overbilling, late billing and unnecessary warnings of debt recovery are among the worst failings.

Ovo - which pays interest on accounts in credit - topped the chart for overall customer satisfaction in a recent survey by energyhelpline, with Scotland-based SSE the only big player in the table's top six.

A Which? poll of almost 10,000 customers also found that smaller providers were "streets ahead" on accuracy of bills, staff helpfulness, value for money and overall efforts at customer service. However, the biggest, First Utility, is no more than mid-table in the service league as assessed by Moneysupermarket.com.

Electricity shoppers are gradually catching on, according to trade group Energy UK, which reported that last month nearly 260,000 households decided to switch, and 49% chose a small supplier.

Many customers are wary about switching to an unknown name - two minnows, Bizz Energy and Electricity4Business, went bust in 2008. But regulator Ofgem ensures households are never without gas or electricity and can appoint a "last resort" supplier if necessary.

Smaller firms (those with under 250,000 customers) do not have to offer discounts to vulnerable customers or participate in green energy schemes, and they may not offer payment options besides direct debit. But this can keep prices low. Cold weather and winter fuel allowances are paid by the Government regardless of supplier.

Nine out of 10 households have said they will be rationing energy use this winter to keep bills down, according to Gocompare.com, with 45% saying they will turn down the thermostat and 24% cutting back on using the dishwasher and tumble dryer to save energy. But only 22% of those surveyed had switched supplier in the last 12 months, while 29% haven't switched for more than three years. Some 41% said they thought that it was "too much hassle", while one in 10 were worried that switching would interrupt their supply.

Jeremy Cryer, energy spokesman at Gocompare.com, said comparison sites made it easy to switch. He said: "It only takes a few minutes to enter your details - which can easily be found on your last gas or electricity bill - and find the right tariff for you."