A List X site is a commercial site (i.e. non-government) on UK soil that is approved to hold UK government protectively marked information marked as 'Secret' or above, or international partners information classified ‘Confidential’ or above.
[1] This changed from 'Confidential and above' with the introduction of the Government Security Classification Scheme.
The term has been used since the 1930s and is equivalent to facility security clearance (FSC) used in other countries.
Locations with this status are those normally involved with defence research and manufacturing that is vital to national security.
This article related to government in the United Kingdom or its constituent countries is a stub.