In practice, this means retaining files for future release that would otherwise have been destroyed as obsolete. The Service, in discussion with the Public Record Office , has incorporated recommendations made by the Lord Chancellor's Advisory Council on Public Records into the criteria it uses for judging which files to retain on historical grounds. Public Record Office staff regularly check files earmarked for destruction. In considering the release of historical papers, the Service must take account of the need to protect former staff and agents. It remains a fundamental principle that the identities of agents must be protected.
Privacy issues are also important.
Aside from these specific requirements, the general principle underlying the Service's file retention policy is that it seeks to retain only those records which will assist it in fulfilling its functions under the Security Service Act. The Service therefore keeps under review whether it might need its older records to fulfil its functions at a later time, such as for future investigations prompted by new intelligence. There is a balance to be struck between the possible intelligence value of retaining files and the need to ensure that files are not kept unnecessarily. This sometimes caused problems, for example in the late 1960s when the Service faced difficulties investigating some spy cases because relevant records had been destroyed. It therefore became the Service's policy to retain records indefinitely.