Dublin-born O2 chief executive Ronan Dunne is to leave the company to be replaced by Mark Evans, the group's parent Telefonica UK confirmed on Friday.
Mr Dunne, who has headed the company since January 2008, will step down at the end of July. A statement from O2 said he would “continued to support the business with its strategic ambitions” until September.
“O2 is a very special brand and business, which has made this an incredibly difficult decision to make,” he said.
The Irish man’s decision to leave the group comes two months after European regulators blocked Hutchison’s bid for O2.
Mr Dunne had previously indicated he would step down as chief executive following a possible merger between the two companies.
It was reported in March that Mr Dunne had been sounded out about leading a possible management buyout of O2, something he refused to deny.
Mr Dunne is the UK telecommunications industry’s longest serving chief executive and is also a member of Telefónica’s SA Executive Committee. His 15-year tenure at O2 saw him grow the customer numbers from 18 to 25 million.
His replacement, Mark Evans, joined O2 in January 2012 as chief finance officer.
Telefonica’s chief executive Jose Maria Alvarez-Pallete paid tribute to Mr Dunne, thanking him for his “outstanding leadership, dedication and achievements.”
“He has led the O2 brand to achieve great things on behalf of its customers, employees and shareholders and built a team of talent to be proud of. I wish him all the best for the future,” he said.
