The service is expected to initially provide mobile to BT’s employees and corporate customers but will eventually extend to consumer customers.
The multi-year agreement with EE was signed yesterday (10 October) but is still subject to final contract approval.
However, it is a chance for the business to offer consumers offers such as packages combining mobile, broadband, fixed line phone and TV.
BT said it "has a strong Wi-Fi presence that it plans to build on", although a spokesman told Marketing that it was still early days in terms of plans for rollout and promotion of consumer mobile.
BT already provides mobile services, primarily to large corporates, the public sector and small and medium-sized enterprises, through a partnership with Vodafone.
It previously ran the Cellnet network from 1986 to 2002, when it was sold off. It now runs as O2 under parent company Telefonica.
A BT spokesman said: "We are excited about this partnership with EE as it will give us lots of options both in the business and consumer markets.
"BT already operates the UK’s largest Wi-Fi network with more than five million hotspots and we are keen to build on that platform.
"Broadband speeds are improving all the time with fibre and 4G and we want our customers to enjoy the fastest speeds possible, whether they are in the home, office or out and about."