The firm said it will close its site in Rayleigh, Essex, cut 75 jobs from its technical support services and remove a further 125 positions from its energy services business. Some 200 other customer service posts will be cut, E.ON confirmed.
Workers union Unison said the firm's customers face substantial bills every month, giving E.ON huge profits and therefore making the cuts unnecessary.
"It is clear that the competitive market for energy has failed," commented Dave Prentis, general secretary of Unison.
"It is time that the government stepped in and took drastic action to bring energy suppliers into line."
Graham Bartlett, managing director of E.ON's retail business, said the changes will enable the company to make further improvements to benefit consumers.
E.ON currently employs over 16,000 workers in the UK and has around 5.5 million electricity and gas customers.
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