Italian oil and gas firm Eni has been fined 290m euros (£198m; $345m) by the country's competition watchdog for blocking rivals in the gas market.
The security of gas supplies has become a key consumer concern
The problems are centred around the access to a pipeline running from North Africa that Eni has granted its rivals.
It has been ordered to increase the capacity for third parties by an extra 6.5 billion cubic metres of gas a year.
The decision comes as Italy had to use emergency gas reserves amid a cold winter and declining Russian supplies.
Eni said it was considering whether to appeal the decision and pointed out that it had recently submitted pro-competition measures to the watchdog.
"The Authority recognised the validity of the works put forward, but nevertheless decided to take action over the delay in carrying out expansion of the Trans Tunisian Pipeline," Eni said.