It comes after a recent report in Spanish newspaper El Pais that cable company Ono had failed to declare an amount of value-added tax in alleged fraud before Vodafone's takeover earlier this year.
Vodafone said that as soon as it became aware of the issue, the company "instigated a forensic audit to investigate the facts relating to the alleged fraud".
"As this matter remains under investigation, we cannot comment further," it said.
The British firm purchased Ono for €7.2 billion ($9.13 billion).
Member comments