Scottish Government Energy Minister Fergus Ewing has demanded an explanation from the UK's top price comparison websites after a newspaper campaign alleged the sites were concealing tariffs that earned them no commission.

Ewing wrote to the companies, saying it was "following press reports that the website which you operate, which has the purpose of providing full, open, transparent advice to consumers about how to save money on their energy bills, has not disclosed all information but that some information as to various companies tariffs has not been disclosed".

He said: "I am writing to ask for your comments on these very serious allegations. If it is the case that this has been your practice, I would ask if you can provide an explanation."

As well as writing to both industry regulator Ofgem and Energy Secretary Ed Davey "asking why the UK Government has failed to act to protect consumers, and when it will close this loophole", Ewing also pressed the price comparison companies to repay customers, telling the sites they had a clear "moral obligation" to provide "compensation payments for the savings that they could and should have achieved had your website displayed the cheaper tariffs".

However, some of the companies named by the newspaper have refuted the allegations of deliberately concealing information.

Jeremy Cryer, energy spokesman at Gocompare.com, said: "­Gocompare.com customers can see all the energy tariffs on the market by clicking a button on our comparison results page. We have never sought to hide the tariffs available or push customers to take one deal over another."

A uSwitch spokesman said: "We are fully accredited under regulator Ofgem's Confidence Code, meaning that our results tables are always ordered by the savings a customer can make in a fair, independent and unbiased way … We do not pre-select a default answer when giving them this choice, nor do we in any way influence what they should select."

A spokesperson from CompareTheMarket.com, said: "To imply that price comparison websites have not acted in consumers' interests is simply untrue. This year, we forecast that we will save our five million customers over £1.2 billion."