The deadline for the investigation, which looks at potential competition failings in the sector and could have huge implications for both energy suppliers and consumers, was originally set for December 25.
However the Competition and Markets Authority (CMA) said it will now extend that deadline by up to six months as a result of the high number of responses received.
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It added that provisional decisions and remedies will now likely be published in January 2016, with its final decision out in April.
Over the summer, the watchdog's provisional findings had highlighted "a range of problems hindering competition in the market, including the extent to which consumers are engaged in the market and shortcomings in regulation".
Explaining the decision to extend its investigation, Roger Witcomb, chairman of the energy market investigation, said: "This investigation - and the measures that could result from it - will have long-standing consequences for millions of customers in an industry of vital importance to the whole country.
"As the most comprehensive investigation into the energy market since privatisation, this is a once in a generation opportunity to shape the future of this market for the better. It's important that we get it right."
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