reported that a June 1915 diary entry made by Walter Kirke, deputy head of military intelligence at GHQ France, stated that Mansfield Cumming, the first chief (C) of the SIS was "making enquiries for invisible inks at the London University".
In an entry made in October he said that he "heard from C that the best invisible ink is semen" because it did not react to detection methods commonly in use and it was readily available.
Intelligence agents thought they had solved a major problem when the heard that semen would not react to iodine vapour.
It was reported that the agent who discovered the use for the bodily fluid was teased so much that he was moved to another department, and that one agent had to be reminded not to use 'ink' which was not fresh because of a problem with the smell.
The revelations are from Professor Keith Jeffery’s book “MI6: The History of the Secret Intelligence Service 1909-1949,” which was just published. Jeffery, of Queen's University, Belfast, was provided with access to all of the MI6 files from the years his book covers.