Energy supplier SSE has been fined £2.06m by the regulator Ofgem for failing to publish inside information about the wholesale energy market.
SSE failed to publish timely information about future availability of its generation capacity, Ofgem said.
This was likely to have had a significant effect on forward wholesale electricity prices, it added.
SSE said it had acted in "good faith", but admitted its actions had failed to meet Ofgem's requirements.
The fine relates to delays in disclosing a deal that SSE struck with National Grid in March 2016.
SSE had said that its three generating units at its Fiddler's Ferry power station were likely to close the following month.
However, on 22 March, it signed a non-binding agreement with National Grid to provide power from the site.
That reversed the likelihood that the units would close, said Ofgem. With a combined generating capacity of 3% of Britain's peak electricity demand, this had a significant impact on demand and supply, affecting wholesale prices.
"The agreement was likely to have a significant effect on wholesale prices, and was therefore inside information," Ofgem added.
"SSE did not publish this information in a timely manner. Instead, it waited until 30 March 2016 to make an announcement once it had finalised the contract."
This resulted in four days' trading without the market knowing that more generation was likely to be available than previously thought.
"It is likely this led to some market participants paying more for wholesale electricity than they should have," Ofgem said.
In response, Martin Pibworth, SSE's energy director, said the company took its market disclosures "extremely seriously and acted in good faith", publishing details of its contract for Fiddlers Ferry power station once signed, which he said was in line with SSE's interpretation of the regulations at the time.
"We subsequently understood that Ofgem's interpretation required disclosure to the market at an earlier stage. We admit that our approach was not in line with this requirement," Mr Pibworth added.
"SSE did not benefit from disclosing only once the contract was signed and remains committed to clear and transparent rules for all market participants. We will be pressing regulatory authorities for additional guidance for market participants going forward."