Over the past 12 months the county council says around 18,000 books and videos, worth a total of £150,000, have disappeared.

But library managers say most of the missing items are borrowed and not returned, rather than a deliberate act of theft.

The usual reason is that people move house and forgetting to notify the local library of a change of address.

The charges for overdue items are also a factor in items not being returned.

Security systems, such as electronically tagging items and close circuit TV, are in operation in some libraries, but librarians do not want to discourage people going in.

In an attempt to combat the problem, a new online joining system has been set up in parts of the county so there is no requirement for proof of identity.

However library managers say they will still be able to check that addresses are valid.