Ofgem's boss said the price rises were having a 'significant impact' on the sector.
Card firms are abusing their dominant market position, said one expert.
Sterling fell to a one-month low on Wednesday as investors pushed back expectations of a rate hike by the Bank of England after a flurry of weak data. In early London trade, the British currency eased 0.1% to $1.3636, with more losses expected after it dropped below a low of $1.364 hit on Monday, according to analysts. Even a subdued greenback and improving risk sentiment after debt-laden China Evergrande calmed nerves by saying it would repay a yuan-denominated bond on Thursday failed to lift the pound.