Customers of 650 of the world's largest firms can now get a daily reminder of their suppliers' green credentials with the launch of new software that allows web users to easily view a score out of 100 for multinationals' environmental record.

The plug-in from Swedish IT start-up BadBuster means that users of Internet Explorer can see a pop-up window with a number rating of a firm's green performance every time they scroll over the company's name. The window is also displayed in green, yellow or red to instantly display the firm's level of environmental performance and offers a link back to BadBuster's site, which shows the various reports the ranking was based on.

The company said that the free downloadable software was currently in beta and would be improved over time, with a plug-in for the Firefox web browser expected before the end of the year and other browser versions scheduled to be released next year.

The ratings are based on an average score taken from various "reliable and independent" publicly available reports into firms' environmental performance, such as the Carbon Disclosure Project and Greenpeace's Guide to Greener Electronics.

The company said the aim of the new functionality was to provide "a simple way for consumers to see how serious companies are about cutting their negative impact on the environment".

However, despite providing an easy way for consumers to check environmental reports on a given company, the approach is likely to face criticism for its simplistic approach to gauging firms' environmental performance.

The number of sources used to generate the 0-100 score varies from firm to firm and BadBuster admits it has not used a weighted average, adding that "we cheerfully ignore the differing stature and prestige of analysts and publications" on the grounds that "we feel that this gives a 'truthier' picture of what sort of environmental quality companies actually have".