Some 370 of the 1,000 employees surveyed by Fujitsu Siemens said they never turned off their computers when they left work.
Fujitsu has timed the release of the report to coincide with the nationwide Energy Saving Week aimed at raising awareness of environmental issues.
Despite 78% of employees feeling that they are more environmentally aware than five years ago, they are ignoring this most obvious and easy energy-saver.
"It was surprising to see that so many workers claimed to have 'gone green' in the workplace and yet they still overlook this most basic way to save energy,” says Garry Owen, head of product marketing at the company.
"UK businesses need to consider both the financial and environmental implications of leaving a computer running and make turning off their PCs each night a policy.”