Ofgem will consult on the design of the safeguard tariff for default deals while the Government's draft bill progresses through Parliament.
The government said in a statement, however, before full draft legislation due later this morning: "The need for it would be kept under review, and extensions could be made, on the advice of Ofgem, up to the end of 2023 at the latest".
But after 192 MPs of all parties wrote to the PM in September complaining that poorer customers were being preyed on by energy firms, Mrs May revived the plan and backed a price cap in her recent Tory conference speech.
The government has yet to clarify whether the cap will be an absolute cap - meaning consumers can not be charged more than a set amount - or a relative cap, whereby the standard variable tariff must not be more expensive relative to fixed tariffs.
"We've got 60 competitors in the market with new entrants approaching 20% market share, we've got high levels of switching, with 3.5 million customers switching so far this year, which is a significant increase on 2016", he told Today.
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The Government said it welcomed the action by Ofgem but believed this could go further to protect everyone on default tariffs "from the unfair practices now seen in the market affecting two-thirds of households in Great Britain".
A Department for Business spokesperson said draft legislation on an energy price cap for default tariffs would be introduced this week.
It will apply to anyone on a standard variable tariff, the expensive plans that customers are moved to when cheaper, fixed deals end.
Business and Energy Secretary Greg Clark said: "The energy market is broken".
Alex Neill, Which? managing director of home products and services, said: "For millions of consumers anxious about their energy bills, a cap might sound like a positive move".
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Ofgem shares the government's concern that the energy market is not working for all consumers and is determined to reduce the detriment suffered by those overpaying for their energy, particularly those who are vulnerable. It's great that we're moving on to think about how, rather than if, we make it happen.
People will be able to save even more money by switching to a better deal.
The regulator said it will work on extending price protection to at least 2m more vulnerable households for winter next year, once the timing of the government's price cap is put in place.
What is a standard variable tariff?
Britain's energy market is dominated by the so-called big six providers - Centrica's British Gas, SSE, Iberdrola's Scottish Power, Innogy's npower, E.ON and EDF Energy, which account for about 85 percent of the retail electricity market. She said her center-right government was trying to fix a market that punishes customers for their loyalty to a supplier.
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