Energy firms should use soaring profits to cut "crippling" bills for consumers, Ed Miliband insisted.
The Labour leader highlighted figures this week that suggested power companies were currently making the equivalent of £125 per customer per year - up from just £15 in June.
Writing in the Sunday Mirror, Mr Miliband warned that rising electric and gas prices were going to cause a "huge winter squeeze on the finances of families and the elderly".
He hit out at Prime Minister David Cameron for merely convening a Downing Street summit on Monday to discuss the issue.
"Ministers in suits around a table in a warm room will do nothing to persuade people that Mr Cameron and this Tory-led government understands the realities of families struggling to pay the bills," the Opposition leader wrote.
"All the big six energy firms have seen their profits rise sharply this year. On Friday, Ofgem said the profit margin for energy companies has risen to £125 per customer per year - up from £15 in June.
"There is nothing to stop those power companies giving up those profits. Instead of an improved bottom line they could use this extra money to stop crippling price rises."
Mr Miliband pointed to the decision by SSE to sell its energy on the open market, rather than merely to its own supplier arms.
He said that was part of the "fundamental reshaping of the energy market" he demanded in his party conference speech last month.
"At present 80% of people pay too much for their energy," Mr Miliband said. "If the few big, dominant firms were forced to sell the power they generate to any retailer, companies such as supermarkets could come into the market."
