The TUC is concerned that although employers are improving their internet security systems, their good work could be undermined if they fail to ensure that their employees' skills are up-to-date.
The 2008 Get Safe Online Report, published earlier this week, revealed that 66% of internet users use the same password for multiple websites.
It also revealed that 23% had posted confidential or personal information online, and that 17% had opened email attachments from an unknown source, potentially putting themselves at risk from viruses or other malicious software.
This could have a huge impact on employers, who could be at risk of losing valuable commercial or customer data, or losing money through damage or downtime due to attacks.
In response, the TUC has teamed up with GetSafeOnline.org and the Department for Innovation, Universities and Skills to produce http://www.worksmart.org.uk/nsfw/ Not Safe For Work?, a free online toolkit for staff.
The toolkit brings together video interviews from industry experts, quizzes and reading lists, and offers a "personalised prescription on internet security issues that is tailored to an individual's internet usage".
The toolkit covers malicious software, identity theft, workers' web rights at work, and privacy online.
As well as being used by UK workers directly, it is hoped that employers will make time available for their staff to use the toolkit.
TUC general secretary Brendan Barber said, "The benefits that the internet brings to UK business are growing every day. Social media and networking are increasingly important to people's professional and personal lives.
"But employers must make sure that staff are aware of the dangers associated with working online. Without training, staff may well find themselves the weakest link in the security chain, without ever knowing what they are doing wrong."