Payments between E.ON and AgeUK are under investigation after it was found that the charity had been promoting the energy provider's “great value” tariffs in leaflets and booklets sent to elderly people.
An investigation by The Sun found that Age UK received around £41 for each person who signed up to the promoted tariff - amounting to at least £6m a year
However, it was found that the tariff cost around £245 more compared to E.ON's cheapest deal. With 52,000 customers on the deal it means E.ON has pocketed a potential £37m extra.
Neither E.ON nor AgeUK have confirmed that a commercial agreement exists.
E.ON said in a statement: "Our current Age UK tariff was the cheapest product of its type in the UK when it was launched in January. Customers can switch between products at any time without incurring any costs […] But in line with Ofgem's rules we can't switch people without their consent."
Age UK denied pushing expensive tariffs and questioned the "interpretation of the figures".
"Age UK has worked with E.ON for the past 14 years, openly and above board, and they have been generous supporters of our charity over and above the number of customers on the tariff. We launched the most competitive, fixed two-year energy tariff available anywhere on the market on January 20 this year, with no exit fees.
"Energy prices change all the time and we have always advised older people to look out for new good deals and we will continue to do so."
However, Energy Secretary Amber Rudd said she was taking the allegations “very seriously” and had contacted watchdog Ofgem to investigate further.
Ofgem said its rules require energy companies to treat consumers fairly when they are marketing and selling energy: “Ofgem has a track record of punishing firms who mislead consumers and we will look at carefully at these claims.”