The PM has vowed to end the "injustice" of rising energy costs.
Stephen Murray, energy expert at price comparison website MoneySuperMarket, says: "For customers who have the ability to switch - the majority outside the most vulnerable - an energy price cap would be a disaster... it will lead to numerous best deals disappearing, prices finding a higher level and a growing market of disengaged customers".
Mr Murray added: "A price cap, whether relative or actual, will lead to numerous best deals disappearing, prices finding a higher level and a growing market of disengaged customers".
Theresa May insisted it was right to intervene to tackle "rip-off" standard variable tariffs but confirmed that the cap set by the regulator Ofgem would rise if suppliers' costs increased.
Yes. In 2013 then Labour leader Ed Miliband said his party would freeze gas and electricity bills for every home and business in the United Kingdom for 20 months if it won the 2015 election.
Mrs May said she will introduce a cap on poor value standard variable tariffs to the benefit of 17 million households, saving families up to £100 a year.
But it is nearly identical to a Labour policy from 2015 which was - at the time - slammed by the Tories as a quasi-Marxist state intervention. "Under our cap the prices will be able to go down".
Mrs May quotes a government backed report last year which found "households are already paying £1.4bn a year more than they need to". Like other providers it relies heavily on these standard variable rate tariffs - about three-quarters of customers are on these lucrative contracts. 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.
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The chief executive of energy trade body, Energy UK, has said the proposed cap on bills "effectively risks giving up on competition at a time when we need engaged consumers more than ever".
"This is the difference that strong leadership under Theresa May can make for ordinary working families".
"Ed Miliband didn't suggest a cap on energy prices", she said. "Unlike the Tories' half-baked cap which only deals with some people, Labour policies will help people on every tariff", a spokesman said.
"Intervention by government will harm investor confidence at a time when the country requires significant investment to deliver energy security and the low carbon agenda", it said in a statement.
He added: "The Competition and Markets Authority's thorough two-year investigation identified low levels of consumers switching energy providers as a challenge, and put forward a range of recommendations to address this".
Bosses at British Gas owner Centrica and ScottishPower have also recently spoken out against the proposal, warning that it could lead to higher bills for consumers and reduce competition.
What do consumer groups think about a cap?