BT is to move its 100,000 employees directly from the Microsoft Windows XP operating system to Windows 7, skipping Vista.
The company has 75,000 desktop computers, as well as 25,000 laptops, currently running XP.
BT beta tested Windows 7 on 200 machines, ahead of the system's release on 22 October, but would not detail the timescale or plans for the overall move. Instead it said saying that at the moment users could upgrade to Windows 7 as they wished.
Peter Scott, BT's chief technology officer for end-user systems, did not disclose why BT was not using Vista. But at the event Steve Ballmer, Microsoft chief executive, conceded that Vista had been met with "vibration" in the market, referring to complaints among some users.
Cost is a vital element of BT's move to Windows 7, according to Scott. BT is attempting to carve over £1 billion from annual operational costs, after problematic IT services contracts with NHS and Reuters are reported to have wiped over £1.6 billion from its books.
The system would "extend the life" of BT's hardware and improve power management, he said, as well as being easy for the user to upgrade to.
Using Windows 7's in-built security features would also remove some of BT's "complexity", Scott said, by eliminating separate firewall and virtual private network security suppliers. "We have too many vendors, too many support contracts and licences," he said. "Now we can use Microsoft's security features and just have one supplier to deal with."
Scott said Windows 7 would provide BT staff with a "better user experience", including faster start-up, compatibility with BT's smartcard VPN access for mobile workers, and stronger search capabilities. It also offered "useful" encryption through the in-build BitLocker tool.
BT was concerned about application and hardware compatibility with Windows 7, Scott said, but he expects this to ease as the software is released and other suppliers tie in their products.
BT also uses Microsoft's Office Communications Server, Exchange and Sharepoint technology, as well as a Citrix network access platform. The company runs 11,500 Blackberrys and 6,500 Windows Mobile based devices.