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Liberal Democrat Energy Secretary Ed Davey warned that a price freeze could result in the lights going out.
"Everyone wants to help with the cost of rising bills, which is why Liberal Democrats have cut income tax by £700 for working people. But Labour's plan is a promise that won't work," he said.
"When they tried to fix prices in California it resulted in an electricity crisis and widespread blackouts. We can't risk the lights going out here too.
"Fixing prices in this way risks blackouts, jeopardises jobs and puts investment in clean, green technology in doubt."
Paul Massara, npower CEO, said: "It's very easy for politicians to come up with simple-sounding solutions to difficult problems.
"But in reality, there are three main factors that influence prices: fixing inefficient housing stock, the investment required to replace the UK's energy infrastructure and the cost of the buying energy on the global market.
"If the Labour Party can commit to reducing policy costs on household energy bills, stopping the smart meter roll-out, preventing commodity cost increases and accept that there won't be any investment in new power stations and infrastructure, then we could freeze our prices.
"But will this make things better for Britain?"
Price comparison website UKPower.co.uk said: "This certainly puts the pressure on suppliers to inflate prices between now and the election as they won't be taking any chances on the voting outcome.
"The funny thing is, though, you can already freeze your energy bill with some tariffs until 31st March 2017 which beats Labour's pledge."
Britain's "Big Six" energy firms revealed profit rises earlier this year following a particularly cold 2012/13 winter:
John Cridland, director-general of the pro-business lobby group CBI, said:
Conservative Party chairman Grant Shapps said a price freeze would actually increase household energy bills over the next two decades.
"While we're forcing energy companies to put their customers on their cheapest tariffs, Ed Miliband would introduce an energy policy that would add £125 to the typical household's bill," he said.
The party said a 2030 decarbonisation target supported by Labour would mean a rise of that scale over the next 17 years.
Energy firm SSE (formerly Southern Electric and Scottish Hydro Electric) said: "Instead of price freezes which will lead to unsustainable loss-making retail businesses, the Labour Party should put policy costs into general taxation, taking them off energy bills.
"This would wipe £110 off the average person's bill and shift the cost away from those who can't afford to pay and on to those who can."
In October 2012, David Cameron promised to compel energy firms to put customers on their lowest possible tariffs.
The Prime Minister's Energy Secretary Ed Davey later admitted that while the policy would make energy bills simpler, it would not necessarily make bills cheaper.
The "Big Six" energy companies said the plan would be very difficult to put into action.
Gas and electricity prices will be frozen for homes and businesses for 20 months after the 2015 general election if Labour wins power, Ed Miliband announced today.
The Labour leader said the move would save the typical household £120 and an average business £1,800 between May 2015 and January 2017.
But the policy puts him on collision course with the "Big Six" energy companies, which stand to lose an estimated £4.5 billion and have not been consulted on the scheme.
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As Ed Miliband clashes with the energy giants on prices, it is worth knowing how your expensive energy bills are actually made up.
A leading competition lawyer says an energy price freeze could leave Labour open to challenges from the courts or the European Commission.