"Muscular" action will be taken by the government against energy firms' "damaging" price rises, a minister has said, but the timing remains unclear.
He claimed that firms "milked" loyal customers through rising default gas and electricity tariffs.
Five of the big six suppliers have announced price rises this year.
Mr Clark said he did not have the power to affect prices directly, but the government could act to alter a "deficiency" in the market.
Major energy firms announced increases in standard tariffs in the last few months, blaming investment requirements, government demands and the falling value of the pound.
They include:
- Npower increased its electricity prices by 15% and gas prices by 4.8% on 16 March
- E.On is due to put up electricity prices by 13.8% and gas prices by 3.8% on 26 April
- Scottish Power raised its electricity prices by 10.8% and gas prices by 4.7% on 31 March
- EDF, combining previous prices changes with a second announcement of rises in June, will raise electricity prices by 18.1%, but gas prices will stay the same
- SSE will raise electricity prices by 14.9% on 28 April, although gas prices will be unchanged
- British Gas has announced a price freeze until August.
BBC News.