Research by the security firm, which was carried out in conjunction with the Ponemon Institute, revealed the costs of security breaches have increased for the third year running. In 2010, the most expensive data loss incurred costs of £6.2m, up from £3.9m in 2009.
The costs include clearing up after the loss, such as replacing equipment and investigating the incident, as well as through lost clients and rebuilding trust with customers.
More than a third (37%) of the data losses were due to a failure of hardware systems or company policy, while 31% were down to negligence, for example employees misplacing USB sticks and laptops.
“The biggest threats we see are the hostile attacks from outside,” Robert Mol from Symantec told the BBC.
“Putting measures in place after the fact is not good enough any more. Companies need to be alert now rather than waiting for the event to happen.”