The Sun newspaper claims E.ON has paid out £6 million a year to Age UK in return for the charity pushing expensive tariffs to the elderly.
But Age UK has rejected the allegations and the "interpretation of the figures".
A spokesman said: "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."
The idea of the process of switching energy providers has been question by Robin Rolls, director of Outlook Trust, a Christian charity working with older people.
"Of course you can switch but you might not want to, you might be scared of it.
"If you're an older person who doesn't use the internet on a regular basis and you happen to have the wherewithal to type into a search engine 'switch energy' and 1.7 million hits come back from your enquiry, you of course would feel a bit bewildered."
"Back in the day you would have just one person that did your electricity and one who did your gas and the bills came in and you paid the bills.
"Now there are comparison websites, some companies don't go for the comparison websites so it's a really confusing area."
Mr Rolls is encouraging churches to do more to help older people become more computer-literate and in turn find it easier to deal with energy bills.
Age UK is facing investigation from the Charity Commission while the government has asked the energy watchdog to look into the arrangement.
The Charity Commission said: "The Commission is aware of concerns raised in the media regarding Age UK's partnership activities with E.ON.
"The Commission is in contact with both Age UK and Ofgem to determine what regulatory role the Commission might have and any action that might be necessary."
The Sun said it found details or the arrangement within Age UK's annual accounts. It claims the charity has been promoting a special E.ON tariff through leaflets and booklets claiming it was "great value" and "helps save energy and money".
It is claimed that the tariff, on average, costs pensioners £245 more than they would pay on E.ON's cheapest deal.
A spokeswoman for E.ON said: "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.
"If a customer is on a fixed tariff and they opt in for a price alert, and if we issue a new tariff that is cheaper, we will automatically notify them of that.
"But in line with Ofgem's rules we can't switch people without their consent."
Currently there are around 152,000 customers on the deal.
Listen to Premier's Antony Bushfield speaking to Robin Rolls on the News Hour.