Councils have lost sensitive data relating to more than 160,000 individuals in the past five years an investigation has revealed.

According to the Bureau of Investigative Journalism, the personal details of 26,000 people have gone missing in the first half of 2011 alone.

Its research, conducted through submitting Freedom of Information requests to all councils, has shown nearly 70% of data losses since 2005 can be attributed to the loss of just five USB memory sticks.

The greatest single breach occurred in 2009 when Birmingham City Council found it had lost track of a USB stick containing the personal details of 64,000 social housing tenants. Information contained included contact details, dates of birth. The council informed all those affected and has since taken steps to improve information security by password protecting USB sticks.

In May, Rochdale MBC reported the loss of an unencrypted memory stick holding personal data and housing information of 18,904 housing tenants.

Subject to an investigation by the Information Commissioner's Office (ICO) - which has fined Surrey, Ealing and Hounslow councils between £70,000 and £120,000 using recently enhanced penalty powers for serious breaches of the Data Protection Act - the council is yet to notify victims or implement disciplinary measures.

A spokesman from the ICO said: 'Local authorities and their staff can have access to substantial collections of often highly sensitive personal information.

'It is therefore important that these organisations have the necessary security measures in place to keep this information secure, as well as meeting their other obligations under the Data Protection Act including making sure that the data held by the authority is not excessive and is only retained for as long as is necessary.'