The Department for Business, Energy and Industrial Strategy states that more than 18 million households in Great Britain are now on a standard variable tariff or other default tariff.
Her announcement last week that the plan would go ahead initially wiped more than 900 million pounds ($1.19 billion) off the value of the two British listed companies Centrica and SSE alone.
In the meantime it said it was introducing new rules to allow suppliers to roll customers coming to the end of their contracts onto another fixed deal instead of a poor value standard variable tariff.
The Prime Minister pledged to cap energy prices during her Conservative party conference speech, saying she would bring an end "to rip off energy prices once and for all".
Britain will publish on Thursday a draft law created to cap consumer energy prices for millions of households, taking action to try and fix a market it says punishes loyal customers.
At present about 18 million customers are on some form of default tariff, which can cost hundreds of pounds a year more than the cheapest deals.
The regulator will, from February, protect a further one million households who receive the Warm Home Discount.
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The bill would require Ofgem to consult and impose the cap "as soon as practicable" after the legislation is passed.
Business and Energy Secretary Greg Clark said: "The energy market is broken. While five million households will see their bills capped from this winter, I want to see every household protected from rip-off bills".
The regulator's headline measure was that it was to extend price protection to one million more vulnerable customers this winter - saving them an average £120 - through its pre-payment safeguard tariff.
"The introduction of further price protections will give time for Ofgem's reforms to work and for smart meters to be rolled out across the country as we move towards a smarter, fairer, more competitive market".
The Government said it had taken the step because the energy market "does not operate in the interests of the majority of consumers", adding: "While we are in favour of free markets, we will always take action to fix them when they're 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".