The authority late on Thursday unveiled changes to the way tariffs are calculated, effectively lowering returns that companies can expect to reap on their investments.
"There is no uncertainty on regulation. There isn't. And there couldn't be any unilateral modification of contracts," authority chief Andrea Camanzi said in a speech prepared for the authority's annual report to parliament.
The new rules envisage a 7.09% return on new investments already planned.
(Reporting by Stefano Bernabei, editing by Stephen Jewkes)