Energy companies must tell consumers if their cheapest deal is sold through another brand under new rules taking effect later this year.
Ofgem said the rule, to take effect from October, will help consumers find the cheapest tariff, regardless of whether a company sells it under its name or via another 'white label' brand.
White labels are organisations that do not hold a supply licence, but work in partnership with a licensed supplier to offer gas and electricity under their own brands.
Search for a cheaper energy deal
Suppliers who operate such arrangements are British Gas with the white label Sainsbury's Energy, and SSE in partnership with M&S Energy and Ebico.
Ofgem introduced rules last year forcing suppliers to tell their customers about the cheapest deal they offer while limiting each company to four tariffs.
However, until now this rule has not extended to white label tariffs.
Ofgem said it wanted to ensure that suppliers "are clear with their customers about their cheapest tariffs, regardless of the brand they use to offer the tariff".
It said the decision on white labels unlocked further opportunities for consumers to engage with the market through well-known household names and innovative brands while keeping the additional protection that the regulator had put in place for consumers.
Ofgem senior partner Rachel Fletcher said: "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."
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