The energy watchdog said it has launched an investigation into a "potentially misleading" offer by Scottish Power promoted during its last price hike.
Ofgem, which is pressing ahead with a proposed shake-up of the industry, said it will look at Scottish Power's £459 savings claim from its "Direct October 2012" offer.
The energy supplier unveiled the offer earlier this month when it said gas tariffs will rise by an average of 19% from August 1 and electricity will go up by 10%.
Earlier this year, the regulator threatened the "big six" with a referral to the Competition Commission if they did not simplify prices and sell off between 10% and 20% of their electricity output to allow smaller firms to enter the market.
Under the Scottish Power Direct October 2012 Offer, the company guaranteed prices would remain a minimum of 1% per year below its standard monthly direct debit gas and electricity prices until September 30, 2012.
But discussing the product, Alistair Buchanan, chief executive of Ofgem, told the BBC that "when you look at the small print it looks very different from that".
He said: "We have been looking at their marketing material in particular, and under our consumer protection powers we are very concerned about some of the claims they have made and so we will be having a look at that very carefully."
Ofgem stressed that an investigation being launched did not mean Scottish Power had breached its obligations.
A Scottish Power spokesman said: "We agree that information about all energy tariffs across the market should be as clear as possible and we will fully co-operate with the Ofgem investigation.
"The tariff in question was a very limited offer with considerably discounted prices, which is now fully subscribed. However, there are a number of similar products still available on the market from competitors. We believe that all figures that have been quoted by us are accurate."
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