Age UK is being investigated by energy watchdog Ofgem over claims that it promoted unfavourable energy deals to the elderly for cash.
The Sun alleges that Age UK accepted £6 million a year from E.On to "push expensive targets to the elderly".
The paper claims that the E.On rate promoted by Age UK saw pensioners pay £1,049 for a year's fuel - £245 more than on E.On's cheapest rate.
Age UK has rejected these claims. A spokesman said: “We strongly reject the allegations and interpretation of figures in this article."
E.On said its tarrifs were priced according to the market.
“We always work to make sure our tariffs, for all customers, are competitively priced and that is further evidenced by the fact that our current Age UK tariff was the UK's cheapest product of its type in the UK when it was launched, a two-year fixed deal, and when we launched our current one-year fixed product, it was also the cheapest in the UK," E.On said.
The Independent has contacted E.On and Age UK for comment.
More than 150,000 customers are on the fixed price deal, according to reports.
Simon Read, the Independent's personal finance editor, said many elderly people prefer fixed rates as it gives them more certainty about bill payments.
Read said it is standard practice for charities to take a cut from referrals.
"The fact that Age UK made money for each referral is standard industry practice," Read said. "Promoting things is fine, selling an unfair deal is not."
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