Sally Davis, the chief executive of BT's Wholesale arm, is set to step down at the end of October after four years over "personal reasons".
She will be replaced by Nigel Stagg, the managing director of the group's small- and medium-sized enterprise division, BT Business.
BT's chief executive Ian Livingston said: "Sally has been akey member of our executive team and so I very much regrether departure."
BT Wholesale provides network services to fixed and mobile phone operators as well as internet service providers, and its customers include BSkyB and TalkTalk. The division's revenues hit £4.2bn last year.
Mr Stagg said the business "is at the heart of BT underpinning many of the services that businesses and residential users take for granted".