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Many others were still part way through a migration off the Microsoft operating system, according to a study by web consultancy Netcraft.com.
Next, LloydsTSB and Legal & General were among those still reliant on NT4 web servers.
LloydsTSB said, "We are addressing the issue and there is a programme in place." Next and Legal & General did not comment on their upgrade plans.
Other UK firms, such as eBay and NatWest, are in the middle of upgrade plans. NatWest is using a mixture of NT4 and Windows 2000 servers to support its website, according to Netcraft.
EBay is in midway through a global upgrade to the newer Windows Server 2003. Its UK arm has a mixture of NT4 and Windows Server 2003 configurations after starting on its upgrade path this spring.
Netcraft said Tesco was the first large UK company to adopt Windows Server 2003, although it still largely relies on Windows 2000.
A Netcraft spokesman said, "NT4 remains the operating system of choice for a surprising number of big brand websites. But with Windows NT4 officially retired, and Microsoft planning to discontinue security patches and support at the year-end, even die-hard enterprises still running NT4 will switch eventually."
Organisations that do not upgrade from NT4 can pay Microsoft for an extended support contract.