A new ruling, which said a college had breached a woman's privacy by secretly monitoring her emails, means employers cannot spy on staff, say legal experts.
Lynette Copland, who works at Carmarthenshire College in west Wales, successfully sued her employer for breaching the Human Rights convention.
She was awarded more than £6,000 by the European Court of Human Rights.
Employment law solicitor Alison Love said if employers were going to monitor emails they must tell their employees.
Still employed by the college as an assistant to the principal, Mrs Copland from Llanelli, had her movements on the campus, as well as the people she emailed and the websites she visited, tracked over a period of 18 months, nine years ago.
She only found out about the surveillance when her step-daughter called to say the college contacted her to enquire about their relationship.
The college's then deputy principal, Philip Wrentmore, instigated the surveillance from around May 1998 to November 1999.
Mrs Copland said there had been a "clash of personalities" with Mr Wrentmore, who left the college shortly after the episode for reasons of ill health and who died in January.
"It has been a long haul but I'm very pleased at the court's decision, I feel I have been vindicated," said Mrs Copland.
"I feel it is now time to move on."
It was argued the monitoring was to determine whether Mrs Copland was using the college's facilities for "personal purposes."
But the European Court of Human Rights ruled that the surveillance without her knowledge "amounted to an interference with her right to a private life".
The case, which was against the college's paymasters - the UK government - was taken to the European Court of Human Rights in Strasbourg by human rights organisation Liberty who acted on behalf of Mrs Copland.
At the time of the offences there was no general right to privacy in English law but the implementation of the Human Rights Act in 2000 legally protected privacy rights in domestic law.
Liberty said the ruling, on 3 April, meant employers would have to make employees aware if their communications could be monitored and there would have to be a good reason for such monitoring in every case.
Liberty's legal director James Welch said: "This judgment makes perfectly clear that employers who spy on their staff are infringing their privacy," he said.
Alison Love, an employment law solicitor for Hugh James Solicitors in Cardiff, said if employers were going to monitor personal communications, it could not be clandestine.
"Employers must make sure they have the appropriate policies with regard to email and internet use in place, that these are communicated to staff and that they are abided by."
A spokeswoman for the college said it was unable to comment on the findings of the European court as the action taken by Mrs Copeland was ultimately against the UK Government.