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Broadly, there are certain justifiable reasons for an emplyer to monitor employees using audio equipment (for instance for training, security etc.). Such monitoring must, however, be reasonable and use of such equipment in the workplace is covered by the Data Protection Act ("DPA"), which means that the employer must adhere to certain principles.
This means that he must obtain the data fairly and lawfully, inform the employees of the recording equipment and only use the data for a specified purpose. The data must also be limited to that which is adequate and relevant and not be held for longer than necessary.
If these are breached or the monitoring is likely to cause substantial distress to staff, then an employee can try to prevent the monitoring taking place by raising a complaint with his employer or by instigating the grievance process or contacting any relevant trade union (if applicable).
If an employee believes that his employer isn't complying with the law, he can also ask the Information Commissioner to assess whether the employer is meeting his obligations under data protection legislation. The Information Commissioner's Office provides specific guidance on data protection at work and the relevant code recommends that audio and/or video monitoring of employees (other than in public / semi-public areas, such as CCTV in car parks) is only likely to be justified for security or safety risks AND only where less intrusive means are unavailable.
Finally, monitoring is much harder to justify in areas which employees would reasonably regard as private and covert monitoring is, as a general rule, rarely justifiable.
Below are link to various web pages providing further specific information on the issue (including CAB, WorkSmart and the relevant ICO codes).
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