Both network managers and companies face fines of up to £5,000 if they fall foul of a forthcoming crackdown by the Data Protection Registrar (DPR).
DPR, Elizabeth France, said last week that next April will see the release of plans for strict guidelines on interception of email, CV scanning, genetic tests and the results of alcohol or drug checks.
France said the plans were needed because "new technology is threatening personal privacy in the workplace".
A network manager for West Sussex Council expressed concern over the proposals: "We have a central mailbox and you need to open some mail to find out who it's for. This is normal for big companies."
Safeway's business solutions manager, Jeremy Wyman, said his company did not have a culture where it spied on its employees.
"We have made employees aware about what is acceptable use on our networks and warned them to be careful about what they put in their emails," Wyman said. "Employees don't generally object to their employers monitoring personal information in other areas - like company phone bills - if it is to prevent people abusing the system."
Lawrence Phillips, a partner at London solicitors, Tarlo Lyons, said that the way around the crackdown was for companies to make it a condition of employment that they have the right to open emails.
"Without it companies will not have the legal right to monitor emails. Some companies may be scanning email without realising it - their email system or firewall may have logging enabled, which effectively amounts to monitoring if details of emails are kept," he said.
Networking company CNS' divisional director Steve Brown said the plans could make things worse because some employers that are not able to get their employees to sign opt-out agreements would stop checking email.
He was concerned that software like anti-virus engines that opened and scanned email could be made illegal under the code.
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