The telecoms giant currently mainly serves businesses and the public sector with mobile phone services, although the new deal is expected to pave the way for BT to target households.
That would take BT into the "quad-play" market, where telecoms firms are able to offer internet, television, home phone and mobile services.
BT, which jettisoned the former BT Cellnet mobile network in 2001, is ditching Vodafone as its mobile "virtual network" operator after the mobile group's buyout of Cable & Wireless triggered a break clause.
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."
The fixed-line operator is currently focused on broadband and phone lines, but using EE's mobile network would allow BT to provide blanket mobile phone coverage without having to build a huge array of masts.
BT recently extended its reach into the TV market with the launch of its own sport channels, after buying the rights to show 38 Premier League football games a season for the next three years.
Earlier this year the group also bought a slice of 4G (fourth generation) spectrum or airwaves for £186.5 million - widely seen as a precursor to the launch of its own super-fast mobile services.
Operators are currently rolling out 4G networks, which offer much faster downloads, uploads and video streaming than 2G and 3G.
BT's mobile service is likely to combine EE's network of masts, which spans 60% of the population with 4G services, with its own wi-fi hotspots.
No details were revealed on what EE will charge to provide the virtual network, or how long the deal will last. The network will also provide mobile services to BT's employees, of which there are 88,000 globally.
O2 had been expected to replace Vodafone as BT's mobile network provider. Vodafone has been the incumbent since 2005.