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Classified information in the United Kingdom, now called Protectively Marked Information, is a system used to protect information from intentional or inadvertent release to unauthorised readers. The system is organised by the Cabinet Office and is implemented throughout central and local government and the critical national infrastructure. The system is also used by private sector bodies which provide services to the public sector.
In 2014, a new classification system, the Government Security Classifications Policy, replaces the old Government Protective Marking Scheme.
Policy is set by the Cabinet Office. The classification system was formerly included in the Manual of Protective Security (MPS) which specified the impact of release and protection level required for each classification. Departments issued localised versions of the content of the MPS as appropriate to their operational needs. The Security Policy Framework (SPF) now supersedes the Manual of Protective Security  and contains the primary internal protective security policy and guidance on security and risk management for HM Government Departments and associated bodies. It is the source on which all localised security policies are based.
In 2013, the Cabinet office issued the Government Security Classifications Policy, which takes effect in 2014. It replaces the old Government Protective Marking Scheme.
The older system used five levels of classification, supplemented with caveat keywords.:Annex One The keyword must be placed in all capital letters in the centre of the top and bottom of each page of a classified document. In descending order of secrecy these are:
Access to protectively marked material is defined according to a vetting level which the individual has achieved.
Vetting is intended to assure the department that the individual has not been involved in espionage, terrorism, sabotage or actions intended to overthrow or undermine Parliamentary democracy by political, industrial or violent means. It also assures the department that the individual has not been a member of, or associated with, any organisation which has advocated such activities or has demonstrated a lack of reliability through dishonesty, lack of integrity or behaviour. Finally the process assures the department that the individual will not be subject to pressure or improper influence through past behaviour or personal circumstances.
Protectively marked material must be accounted for in a manner appropriate to its classification level and disposal must be in accordance with the SPF. The act of destruction or disposal is included in the accounting process.
Protectively marked material may also be marked with a descriptor, or privacy marking, which identifies sensitivities around distribution and handling.
Examples of descriptors include, but are not restricted to:
Protectively marked material may bear a nationality caveat, a descriptor defining which nationality groups it may be released to. By default material in the UK is not caveated by nationality, the classification being sufficient protection.
Examples of nationality caveats include, but are not limited to:
Dissemination of already protectively marked material may be further limited only to those with a legitimate need to know using compartmentalisation by use of codewords. Examples of compartmented material would include information about nuclear warheads, fusion, and naval nuclear propulsion. In some cases the existence of a codeworded compartment is itself classified.
Examples of codewords include, but are not limited to: