Energy firms must tell customers about cheaper tariffs available from their other brands under new rules taking effect this autumn.
Regulator Ofgem says the rule, which is due to come into effect on 1 October, will help people find the cheapest tariff, no matter whether it's sold under the company's own name or via its 'white label' brand.
White label providers don't hold their own supply licence, but work in partnership with a licensed supplier to offer gas or electricity under a separate brand.
Examples include British Gas with the white label Sainsbury's Energy, and SSE with the white labels M&S Energy and Ebico.
Join MoneySavingExpert.com's Cheap Energy Club to start cutting your bills now.
Ofgem introduced rules last year ordering suppliers to tell their customers about the cheapest deal they offer. However, this rule didn't apply to white label tariffs (see the Energy firms set to tell you about even more cheaper deals MSE News story).
'Transparency is important in rebuilding consumer trust'
Ofgem says it wants to ensure suppliers "are clear with their customers about their cheapest tariffs, regardless of the brand they use to offer the tariff".
Senior partner at Ofgem, Rachel Fletcher, says: "Selling energy through white label brands has the potential to increase consumer choice and engage consumers to shop around through well-known brands.
"But it is important that consumers are given the complete picture about all their supplier's tariffs. We are acting to reduce barriers to white labels entering the market and to ensure suppliers tell their customers what the cheapest deal is for them, whatever brand it is marketed under.
"Transparency about the cheapest tariff that a supplier offers is important in rebuilding consumer trust in the market."
Ofgem has also confirmed that from 1 October, white labels will be able to offer four core tariffs, alongside their parent company's own four core tariffs.
As of December 2013, suppliers are only allowed to offer four core tariffs per fuel to make it easier for customers. At the time, a temporary measure was also imposed on white label suppliers created after 1 March 2013, so their tariffs were included as part of the four core tariffs offered. White labels that existed before 1 March 2013 were not subject to the cap.
The regulator also says there won't be a limit on the number of white-labels a supplier can have.
Additional reporting by Helen Knapman.