The forthcoming competition inquiry into the UK's energy companies should also investigate the wholesale gas market, a senior MP has warned.
Tim Yeo, chairman of the Energy Committee, has written to the Competition and Markets Authority (CMA), which is running the inquiry.
He said the investigation would be "compromised from the outset" if it did not look at wholesale gas prices.
The CMA said it would be responding directly to Mr Yeo.
"I am surprised that the CMA has suggested that it is 'not minded to investigate' the wholesale gas market, and I am writing to urge it to reconsider," wrote Mr Yeo.
He said that gas was the main method for heating homes and was the dominant fuel for powering the UK's electricity network.
The study of the UK's big six energy firms was launched after the regulator, Ofgem, questioned the effectiveness of competition in the market.
Ofgem has also expressed concern about wholesale prices.
Earlier this year it wrote to energy suppliers, to ask why lower wholesale gas prices in the winter had not been passed on to consumers.