A statement from Nokia said, as part of the deal, it would provide equipment and services at BT radio sites across the UK, adding that this would make it BT's biggest equipment provider.
The portion of BT's network that uses Nokia gear includes Greater London, the Midlands and rural locations. This is to be extended to cover a number of other towns and cities.
"Nokia will supply its AirScale Single RAN portfolio for both indoor and outdoor coverage, including 5G RAN, AirScale base stations and Nokia AirScale radio access products," the company said.
"The deal will also see Nokia optimise BT's 2G and 4G networks and work alongside BT on the development of the OpenRAN ecosystem."
In a reference to Chinese telecommunications equipment vendor Huawei Technologies, which has now been excluded from British 5G networks, Nokia said: "This enhanced Nokia footprint will support BT's commitments to the UK Government around the use of High Risk Vendors in UK network infrastructure."
Huawei was initially given the okay to bid for supplying gear for non-core portions of networks in the UK and its gear was used by all four companies that have rolled out 5G. However, under US pressure, London back-flipped in July and said all Huawei equipment would have to be removed by the end of 2027.
The Nokia announcement said BT "would also use Nokia Software's ng-SDM and NetAct network management platform, supporting the network evolution to 5G. These build upon the existing network architecture and provide an immediate cornerstone and single platform for new 5G-based services. Nokia will also provide its state-of-the-art Cell Site Gateway product providing key backhaul connectivity".
"Nokia will also provide digital design and deployment for a faster time to market as well as optimisation and technical support services."
Nokia president and chief executive Pekka Lundmark said: "I am delighted that BT has extended its partnership with Nokia on 5G RAN, making Nokia BT's largest infrastructure partner.
"Our two companies have collaborated for over a quarter of a century in order to deliver best-in-class connectivity to people across the UK. We are proud to support BT's 5G network evolution and look forward to working even more closely together in the years to come."
BT Group chief executive Philip Jansen said: "Digital connectivity is critical to the UK's economic future, creating jobs and underpinning sustainable growth. That's why BT is making game-changing investments in full fibre and 5G.
"In a fast-moving and competitive market, it's critical we make the right technology choices. With this next stage of our successful relationship with Nokia we will continue to lead the rollout of fixed and mobile networks to deliver stand-out experiences for customers."