The CEO of O2 is leaving the company
The telecoms executive will be replaced by O2 CFO Mark Evans, who joined O2 in 2012 after working for the likes of Vodafone and ntl. Evans also oversees giffgaff, which is a wholly owned subsidiary of O2 parent company Telefonica.
The reason for Dunne's departure is currently unknown.
Dunne said in a statement: "The decision to reconsolidate Telefónica UK represents an important and positive milestone for the business and marks the beginning of an exciting new chapter for O2.
"O2 is a very special brand and business, which has made this an incredibly difficult decision to make. We have built an outstanding team to take the business forward and in Mark, Telefónica has chosen a colleague and leader with outstanding commercial acumen and a drive to win. I wish him, O2 and Telefónica every continued success."
José María Álvarez-Pallete, executive chairman of Telefónica SA, said that Evans has been "instrumental" in delivering revenue and profit growth since he joined the company.