KPMG Europe LLP has signed a five-year deal worth £62m which will see BT upgrading its data networks.
The deal will provide an IP-based telephone system and other improvements aimed at delivering significant operating-cost savings to KPMG businesses, which it said would ease future expansion and improve employee productivity.
BT will be responsible for KPMG's wide area network and local area network refresh and management, deployment of Cisco IP telephony, integration with an existing Microsoft Office Communications Server platform, video and audio conferencing.
It will initially take over the infrastructure in KPMG's UK and German divisions but this could be widened to elsewhere at a later date.
Bryan Clark, the KPMG partner leading the IT infrastructure consolidation, said in a statement: "This is an ambitious outsourcing programme and one that will deliver significant benefits to both our cost base and our effectiveness in serving our clients.
"This deal will also assist and support our ambitious expansion plans. Having a consistent infrastructure platform and service model will help facilitate the integration of additional KPMG firms joining our newly created European firm."