Boris Johnson "Miliband says he will imitate the catastrophic policies of the emperor Diocletian, by imposing a price freeze on energy bills for the 20 months succeeding the election".
She has promised a cap on standard variable tariffs in the Tory election manifesto.
It said it is often those who can least afford it, such as pensioners and people who do not earn much money, who are stuck on these high bills.
The new plan was first announced last month and many industry figures do not agree with it as they believe it could lead to higher prices.
The Conservative leader vowed to place a cap on standard variable tariffs to remove the "injustice" of soaring energy costs.
"Gas and electricity bills only ever seem to go in one direction, eating up more and more of your monthly pay packet".
Tories say the price limit could save 17 million families £100 a year, but energy bosses warned the Tory price cap could undermine competition.
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A number of Tory MPs favouring free market policies, including some at senior ministerial level, feel the plan is far too interventionist for a Conservative government, and are aiming to water down the proposal in the next parliament.
Writing for The Sun, Ms May wrote: "It is clear to me that the energy market is not working for ordinary working families".
But Stephen Fitzpatrick, the chief executive of smaller supplier Ovo Energy, described the policy as a "bold and ambitious move" - and argued that a cap would not harm consumers or competition but rather offer customers a safety net to protect them from overcharging.
She told Sky News: "With about two-thirds of consumers sitting on the expensive standard variable tariffs the Tories are talking about, there are big savings to be made by making a whole of market comparison". They raise the prospect of prices across the board tending to be closer to the Ofgem-set cap, thereby reducing the incentive for consumers to switch.
However, energy analysts said that people would see their bills go up by £50 a year because of increasing energy policy and network costs that are already locked in over the course of the next parliament. "A cap suggests a maximum amount that can be charged, not a promise that bills won't go up year on year".
Shares in Centrica have fallen 15% since the start of this year and some are speculating that it may cut its dividend to shareholders to cope with an estimated £200 million hit to profits from the cap. Josh Hardie, its deputy director general, said: "A major market intervention, such as a price cap, could lead to unintended consequences, for example, dampening consumers' desire to find the best deal on the market and hitting investor confidence".
Meanwhile Alex Neill, managing director of home products and services at consumer group Which?, says: "Measures that significantly reduce the cost of your bills overnight will be welcomed, but this can not result in an increase in bills overall, undermine improvements in service or bring much needed innovation to a halt".