Almost a third of large companies in the UK and the US have admitted to hiring snoops to read and analyse employees' emails in a bid to mitigate legal, financial or regulatory risks.

A study of more than 400 companies with over 1,000 employees found that companies are increasingly employing staff to read outbound emails. The study was conducted by Forrester Research for email monitoring firm Proofpoint.

The top concern is protecting financial privacy and the identity of customers, followed by compliance issues and the prevention of information leaks. Companies ranked monitoring for 'inappropriate content' and 'attachments' as less important.

"It is not something that is broadcast," said Gary Steele, chief executive at Proofpoint. "There are organisations where employees think they can say whatever they want to say and nobody is going to read it."