E.ON has said it will compensate customers it may have mis-sold to. Find out how to make a claim.
E.ON boss Tony Cocker has apologised after the energy firm was found to be misleading customers, according to an investigation by Ofgem.
Some 94,000 E.ON customers trying to switch providers were charged new prices during the 30-day window after a price rise.
ITV News Economic Editor Richard Edgar has tweeted:
Eon feels the heat as it's fined £12 million by the regulator for misselling contracts as recently as December last year.
Eon will pay its vulnerable customers (who get Warm Home Discount) about £35 each - there are about 1/3 of a million of them
Eon misled customers and its management failed to put checks in place to stop this.
Energy supplier E.ON is to pay £12 million to vulnerable customers as part of a redress package after an Ofgem investigation found the company broke energy sales rules between June 2010 and December 2013
The Co-operative Energy firm has announced a price rise of 2.5% on average.
In an email to customers, the company said: "As winter sets in and some energy companies raise their prices by as much as 10%, we’ve got some good news for you to warm to.
"Co-operative Energy is raising its prices on 8 January…but only by 2.5% on average across all our customers."
Energy Secretary Edward Davey called E.ON's price rises announcement "disappointing news".
Bur Mr Davey stressed:
This rise is ... lower than it would have been as a result of Government action to reduce the impact of price rises on consumers.
As part of their announcement today, E.ON have confirmed they will pass on these savings to their customers.
This does not let energy companies off the hook.
Asked about the timing of E.ON's price rises announcement, which came amid media focus on the death of Nelson Mandela and the devastating storms across the UK, Downing Street said, "It is for them to explain their decisions."
E.ON has announced its customers' dual fuel energy bills will increase by an average of 3.7% from January 18.
The company said that means the average variable dual fuel bill will go up by £48, electricity only prices will increase by 3.7% or £20, and gas only bills will climb by 4.6% or £37.
Chief executive Tony Cocker said changes announced by the Government earlier this week reduced the overall level of the rise that was necessary to cover extra costs.
The energy firm said it was "working hard to limit the impact on its customers" by announcing a lower average percentage rise than any other major supplier.
Earlier this week, E.ON said it did not expect to have to raise prices in the next 18 months "as a result of social or environment obligations".
But E.ON also warned, "There remains a risk, however, that increases in network charges or wholesale energy costs for example could force a price increase".
The announcement followed the Government's shake-up of green levies on Monday.
Energy supplier E.ON will increase prices by an average of 3.7% from January 18.
Energy firm E.ON said it does not expect to have to raise prices in the next 18 months "as a result of social or environment obligations".
E.ON said in a statement that the Government's energy announcement "should mean our customers pay less next year".
"There remains a risk, however, that increases in network charges or wholesale energy costs for example could force a price increase", it said in a statement, adding, "But the company very much hopes that situation does not occur".
Tens of thousands of pensioners will face a hike in energy prices after E.ON withdrew its discount tariff for over-60s.
The StayWarm deal, which offered fixed-price energy for older customers based on their average consumption levels, will come to an end from 7 October.
The energy firm says the move has been forced by changes to the number of tariffs permitted by regulator Ofgem.
A spokesman said: "We are writing to all customers as their contracts come to an end, and where possible we are also contacting them by phone with the aim of speaking to each customer to help ensure they switch to the best product for their needs."