Energy firms will face a price cap on "rip-off" bills under plans announced by Theresa May.
The Prime Minister said draft legislation for the measure would be published next week as she accused firms of punishing loyal customers.
Mrs May said the Tories would always take action to fix "broken" markets and tackle "monopolies and vested interests".
In her keynote speech to the Conservative Party conference, she accused energy giants of hiking bills for customers who do not shop around for a better deal.
She said: "While we are in favour of free markets we will always take action to fix them when they are broken.
"We will always take on monopolies and vested interests when they are holding people back.
"One of the greatest examples in Britain today is the broken energy market.
"The energy market punishes loyalty with higher prices and the most loyal customers are often those with lower incomes, the elderly, people with lower qualifications and people who rent their homes.
"Those who, for whatever reason, are unable to find the time to shop around.
"That's why next week this government will publish a draft bill to put a price cap on energy bills, meeting our manifesto promise and bringing an end to rip-off energy prices once and for all."