Big Six energy company E.ON has paid millions of pounds to a charity in return for expensive tariffs to be pushed on elderly people, it has been claimed.
The Sun newspaper reports it found details of E.ON payments to Age UK in the charity's annual accounts - amounting to at least £6 million a year.
Energy Secretary Amber Rudd said energy watchdog Ofgem has been asked to investigate.
The Sun claims about E.ON and Age UK:
Age UK had been recommending a special E.ON tariff which on average costs pensioners £245 more than E.ON's cheapest deal
Age UK received around £41 for each person who signed up to the tariff - amounting to at least £6 million a year
There are 152,000 customers on the deal - meaning a £37 million overspend by pensioners
It is claimed the charity had been recommending the special E.ON tariff in leaflets and booklets, stating it was "great value" and "helps save energy and money".
The Sun reports the tariff costs pensioners on average £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."
Declining to comment on the "sensitive" nature of the amount paid to Age UK, E.ON did confirm there was a "commercial relationship" between it and the charity.
Age UK said it "strongly rejects the allegations and interpretation" of the figures. A spokesperson added that a typical commission to Age UK from E.ON is £10 a customer.
In a statement, 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.”