Energy price comparison sites will have to become more transparent, under new rules announced by the regulator Ofgem.
Some of the websites have been criticised for not showing the cheapest tariffs, and not telling customers which deals they earn commission on.
Those that are members of Ofgem's consumer confidence code will have to make changes by the end of March.
Several of the sites affected, including MoneySuperMarket and Uswitch, have welcomed the new rules.
From March such websites will have to show all the tariffs on offer, unless customers choose to see a more limited range.
Many price comparison sites currently conform to the rules by asking consumers whether they want to "switch today".
If they click "yes", they are only shown deals which the site makes money on.
If they click "no", they are shown a wider range of tariffs, which involve customers applying to the energy provider directly.
But Ofgem is making it clear that being asked such a simple question will no longer be acceptable.
Customers will have to be told directly which deals the company will earn a commission on, and which they will not.
"Comparison sites are a great place to start energy shopping, but customers need to feel confident that the sites are providing information they can trust," said Rachel Fletcher, Ofgem's senior partner.
Energyhelpline, a member of the consumer confidence code, said the change could force some operators to leave the scheme.
"The new rules are so tough that some comparison sites are likely to drop out of the code, fearful that to follow it will mean their businesses become unviable," said Mark Todd, director of Energyhelpline.
MoneySuperMarket said it had already made some alterations to its site.
"We have already updated our website to show all of the tariffs available on the market, regardless of whether we have a commercial agreement in place or not," said Stephen Murray, energy expert at MoneySuperMarket.
Uswitch, too, said it would comply with the rules.
In a separate development, Ofgem has announced an investigation into the power company SSE.
The regulator is to examine whether the company put its competitors in the electricity connections market at a disadvantage.
Ofgem has also announced measures to improve competition between companies responsible for local electricity distribution.