A new rule means gas and electricity customers must be given 30 days' notice by suppliers before prices are raised as energy watchdog Ofgem wants households to have more time to switch providers before bills go up.
Energy companies were previously allowed to write to customers up to 65 days after raising prices.
With the new rules from Ofgem, customers must also receive 30 days' notice of any change to their contract which will leave them significantly worse off.
All the "big six" energy providers have brought in price hikes in recent months, blaming the rising cost of gas and oil on the wholesale markets. When the change was announced, Andrew Wright, Ofgem's senior partner for markets, said the changes would "show that we are serious about making sure suppliers play it straight with consumers".
Hannah Mummery, energy expert for Consumer Focus, said: "Ofgem has also made some strong recommendations from its energy market review which should also make it easier for customers to compare energy tariffs and switch to a better deal."
Several energy companies have already started giving customers more notice since the proposals were put forward last year.