When marking a document containing information from Great Britain, which banner marking is correct?

The correct banner marking for a document containing information from Great Britain is SECRET// FGI GBR. This marking indicates that the document contains classified information that is deemed to be of a "SECRET" level concerning British government information. The use of the "FGI" (Foreign Government Information) designation highlights that the document includes information originally provided by a foreign government, in this case, Great Britain (GBR).

When handling foreign government information, it is crucial to appropriately reflect the classification levels as determined by the originating government, which in this instance is represented through the "SECRET" classification. This ensures compliance with both domestic and international regulations concerning classified information management, protecting the sensitive information and maintaining proper handling protocols.