Security vetting in the United Kingdom

In the United Kingdom, government policy requires that staff undergo security vetting in order to gain access to government information.
The government uses four levels of personnel security controls depending on the level of assurance required. Three of these levels are types of national security vetting clearance.
Vetting is intended to assure government bodies 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, an 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.
Vetting is usually carried out by United Kingdom Security Vetting (UKSV). UKSV was created in January 2017 by combining DBS National Security Vetting (DBS NSV) and FCO Services National Security Vetting (FCOS NSV). This change was an outcome of the Strategic Defence and Security Review 2015.
Types of personnel security check.
Baseline Personnel Security Standard (BPSS).
The Baseline Personnel Security Standard (BPSS) checks are normally performed when a person is recruited.
All those with access to government assets are subject on recruitment to the requirements of the Baseline Personnel Security Standard. This includes all applicants for employment in the civil service and armed forces and applies to both permanent and temporary staff and private sector employees working on government contracts, with access to government assets.
The Baseline Personnel Security Standard requires the verification of the following four elements:
A reasonable account of any significant periods (a total of 6 months or more in the past 3 years) spent abroad. Prospective employees who have recently come to the UK or lived abroad may be asked to provide overseas police certificates of good conduct.
BPSS
Employers may initiate the following incremental national security vetting checks on recruits after performing the BPSS check.
Counter Terrorist Check (CTC).
A Counter Terrorist Check (CTC) is required for individuals who are employed in posts that: 
The process for CTC clearance includes:
A CTC clearance must be formally reviewed after ten years (five years for non-List X Contractors).
In the transport sector security vetting requirements, including for Counter Terrorist Check, is regulated by the Department of Transport.
A CTC is required for police officers and many associated staff.
Security Check (SC).
A Security Check (SC) is the most widely held level of security clearance. SC is required for posts involving regular and uncontrolled access to SECRET assets and/or occasional, supervised access to TOP SECRET assets, and for individuals who:
The process for SC clearance includes:
Checks may extend to third parties included on the security questionnaire.
An SC security clearance must be formally reviewed after ten years (seven years for non-List X contractors) or at any time up to that point at the discretion of the vetting authority.
Enhanced Security Check (eSC).
This is similar to an SC but also includes a financial questionnaire. 
Developed Vetting (DV).
DV is the most detailed and comprehensive form of security clearance in UK government. It is needed for posts that require individuals to have frequent and uncontrolled access to TOP SECRET assets, or require any access to TOP SECRET codeword material. It is also required for individuals who: 
The process for DV clearance includes:
A DV security clearance must be reviewed every seven years or at any time up to those points at the discretion of the vetting authority.
Enhanced Developed Vetting (eDV).
Enhanced Developed Vetting requires additional in-depth interviews, which may be necessary for certain roles.
STRAP Clearance.
STRAP Clearance is an additional clearance granted to certain holders of Developed Vetting and Enhanced Developed Vetting clearance, allowing them to access STRAP code-word material. While not officially publicly acknowledged, it is mentioned by UK government bodies on occasion in policy and procurement documents. An individual with DV clearance has to apply for access to STRAP. A decision, based on their STRAP application, is then made on whether or not they are granted access to STRAP. 
Other UK clearances.
In addition to national security clearances, other types of roles and organisations stipulate a need for clearances, including:
Change of personal circumstances.
A change of personal circumstances (CPC) questionnaire has to be submitted when an SC, DV or CTC clearance holder is "marrying, remarrying, entering into a civil partnership, setting up a stable unmarried relationship which includes living with someone as a couple". DV clearance holders also have to report the arrival of new "co-residents" such as a lodger or flatmate. When two cleared persons marry both have to submit CPC questionnaires.
Transfer of a clearance.
A request can be made to transfer national security clearances between organisations, providing they have not expired. Transfers are requested by the "new employing sponsor". Transfers can be the same level of clearance or lower. No more than twelve months must have elapsed since the holder left the organisation for which the clearance was originally granted and no more than six months living overseas. New completed change of circumstances questionnaires, to bring the UKSV records up to date, may be required. The new sponsor reviews the details of the clearance and decides if it is acceptable for the specific new role.