Mark Ions allegedly used his LinkedIn network to approach clients for his own rival agency called Exclusive Human Resources, which he set up three weeks before resigning from Hays last year.
LinkedIn, which has over 20m users worldwide, is widely used by recruitment firms seeking job candidates.
Ions, who worked for Hays for six and a half years, told the High Court in London that he had been a member of for over a year, with the encouragement of Hays.
Hays alleged that Ions breached his contract of employment by using the confidential information about clients and contacts for his own venture.
Ions' solicitor argued that once Ions' LinkedIn invitees accepted his invitation to become "connections", the information became available on his network and was therefore not confidential, but publicly available, at least to his other connections.
However, Justice Richards dismissed that argument and said that even if Ions uploaded the email addresses onto the site with Hays' authority, it is "difficult to imagine that the authority was not limited to using them in the performance of his duties as an employee of Hays."
Ions was ordered to disclose his LinkedIn business contacts requested by Hays and all emails sent to or received by his LinkedIn account from Hays' computer network.
He was also ordered to disclose all documents, including invoices and emails that showed any use by him of the LinkedIn contacts and any business obtained from them.