(Adds Sorgenia statement)
MILAN, Oct 6 (Reuters) - Italy's competition regulator has opened a probe into utility Enel and smaller power company Sorgenia alleging they overcharged grid operator Terna for services relating to the power dispatching market.
The regulator said the probe would conclude by May 30, 2017.
Italian energy watchdog AEEGSI, which signaled the alleged offence to the antitrust, has estimated Terna paid 320 million euros ($357 million) more for the services in the first six months of 2016 compared to the previous year.
Such costs are passed on to consumers through energy bills, it said.
In a statement, Enel denied it had abused a dominant position on the dispatching market.
"Enel believes it acted legitimately and perfectly in line with competition regulations," it said.
Later on Thursday Sorgenia said in a statement it had respected the rules and added it had offered its complete cooperation to the competition regulator over the probe. ($1 = 0.8961 euros) (Reporting by Stephen Jewkes; Editing by Alexandra Hudson)