Brit spies used semen to write top-secret letters

British spies used semen to write top-secret letters, it has emerged.

Spooks at MI6 discovered the bodily fluid could be used as an invisible ink, as it glows brightly under UV light but does not react to detection chemicals.

A diary entry written during the First World War by a senior member of the Secret Intelligence Service describes the tests in detail, reports The Sun.

In June 1915, Walter Kirke wrote that Mansfield Cumming, the first chief of the SIS, was "making enquiries for invisible inks at the London University".

Later he "heard from C that the best invisible ink is semen", which did not react to usual methods of detection.

Frank Stagg, a staff member of Cumming, said he would never forget his bosses' delight when the Deputy Chief Censor said one of his staff had discovered that "semen would not react to iodine vapour".

The find didn’t, however, come without problems. While the agent who made the discovery had to be moved from his department after becoming the butt of jokes, another had to be reminded to use only fresh supplies of semen when correspondents began noticing an unusual smell.

The revelations are included in new book MI6: The History of the Secret Intelligence Service 1909-1949 by Professor Keith Jeffery.