The UK's "big six" energy suppliers are facing a crackdown after a watchdog found that a lack of competition is resulting in excessive bills.
The Competition & Markets Authority (CMA) has unveiled a range of measures to tackle market problems that it says are limiting competition and leaving consumers paying about £1.7bn more than necessary.
The regulator says about 70% of domestic customers of the "big six" are still on more expensive standard variable tariffs (SVTs) rather than cheaper fixed-term deals.
It believes households and businesses could save more than £300 on average by switching suppliers or packages.
The CMA has proposed creating a database controlled by industry regulator Ofgem of "disengaged" customers who have been on SVTs for more than three years, which will allow rival suppliers to target them.
It is also suggesting a temporary safeguard price control to protect customers on pre-payment meters, whose options are more limited, reducing their bills by about £300mln a year in total.
Shares in British Gas owner Centrica PLC (LON:CNA) jumped 4.3p, or 1.9%, to 231.3p as investors digested the news.
Centrica said in a statement that it disagreed with the CMA's assertion that consumers may have been overpaying by about £1.7bn a year.
Chief executive Iain Conn said: "The CMA’s proposed remedies are now subject to consultation. We believe that, provided they are implemented thoughtfully, the majority will benefit our customers.
"We will work through the full report once we have it and submit a formal response in line with the required timetable."
Juliet Davenport, founder of independent supplier Good Energy Group Plc (LON:GOOD), said: "There are some good measures proposed which will mean customers get a better deal."