The number of people changing their gas and electricity company hit 5.1 million in 2007 - a five-year high, according to energy regulator Ofgem.

The figure is up from approximately 4.8 million switches in 2006.

Ofgem said Scottish and Southern Energy had overtaken E.On as Britain's second biggest energy supplier after British Gas in an example of "vigorous competition" in the market.

Customers who have yet to switch could still save an average of £92 a year, despite all the major suppliers increasing their prices, according to the regulator.

Ofgem chief executive Alistair Buchanan said: "The number of customers switching supplier is increasing and five million account switches alone in 2007 shows that suppliers who don't offer competitive prices and good service will lose customers.

"However, some customers have voiced concerns about the market and we have responded by launching an investigation to examine whether it is working effectively for all customers."

Suppliers were continuing to compete by providing tariffs to suit customers, including fixed-price deals, online account deals and green electricity tariffs, which together amounted to around nine million accounts to date.

Research had found pensioners, the unemployed and those on low incomes were less likely to switch than other groups.

In response, Ofgem has launched a pilot campaign called Energy Best Deal with Citizens Advice to encourage more of these customers to switch to a better deal.