FONE Group is suing Telecom Italia for abusing its dominant position in Italy, seeking damages of more than €1bn (£868m).
A Vodafone spokeswoman said the civil action in Milan stated that Telecom Italia committed a series of abuses from 2008 to 2013 “with the intention and effect of impeding growth in competition in the Italian fixed-line market”.
Telecom Italia rejected the claim, saying it was confident it would demonstrate the “total correctness” of its behaviour.
In May, Italy’s competition authority fined Telecom Italia about €104m for abusing its dominant market position as owner and manager of the country’s largest fixed-line telephone network.
Telecom Italia has said it would appeal against the antitrust fine, which was decided after a three-year investigation prompted by rivals Wind, Italy’s third-largest mobile operator, and broadband company Fastweb, a unit of Swisscom.
Vodafone said Telecom Italia had caused a loss of customers, restricted Vodafone Italy’s ability to grow its fixed-line business, and forced it to pay artificially high costs to compete in the market.
Vodafone Italy, which filed the lawsuit, is 77 per cent owned by Vodafone.